The agency's decision puts the Fukushima crisis on the same level as the 1979 accident at Three Mile Island in Pennsylvania.
France's Nuclear Safety Authority rates the Fukushima crisis at six on the scale. The Chernobyl disaster is put at seven, the highest.
Level 5 indicates 'an accident with wider consequences', according to the International Nuclear Event Scale (INES), while level four means there has been an 'accident with local consequences'.
The April 26, 1986 explosion at the Soviet atomic power plant in Chernobyl was the world's worst nuclear disaster.
Unleashed by an unauthorised technical experiment, it spewed radioactive dust over swathes of Ukraine, Belarus, Russia and western Europe. The death toll ranges from a UN 2005 estimate of 4,000 to tens or even hundreds of thousands, proposed by non-governmental groups.
It's like death sentence: Japan n-plant worker's daughter
The families of experts battling to cool Japan's Fukushima plant are a worried lot, with a worker's daughter saying: "My father says he has accepted his fate much like a death sentence."
The Sun quoted a message that was sent by another nuclear worker to his wife that said: "Please continue to live well, I cannot be home for a while."
The Fukushima nuclear plant was badly damaged in the March 11 earthquake and tsunami that devastated Japan. There have been explosions at three reactors while a fire engulfed a fourth reactor, sparking fears of a nuclear meltdown.
A group of experts have been working round the clock to cool the reactors and Thursday helicopters dumped water over them. Twenty people have been confirmed to have suffered from radiation exposure.
The Sun reported that a poignant note was disclosed when there was a rise in the radiation level.
There are about 180 personnel working round the clock to cool the plant. Working in rotating teams of 50, they enter the radiation hotspots for only 15 minutes at a time to limit their exposure.
Radioactive material could reach California today: report
Small amount of radioactive material, blowing from Japan's damaged nuclear power plants, is expected to reach California today but experts have said the radiation would be 'within safe limits' and poses no major risk, according to a media report.
Radioactive isotopes are being blown toward North America 'high in the atmosphere over the Pacific Ocean' and will reach California as soon as Friday, a report in the Los Angeles Times said.
"The material should begin showing up on the (US) West Coast as early as Friday, though it could take up to an additional week for the 5,000-mile trip from Japan to Southern California," the report said.
US network of sensors, known as the 'Radnet' operated by the Environmental Protection Agency and comprising 100 radiation monitors across the country, are monitoring the level of radiation that is flowing from Japan towards the US west coast.
The EPA said it was monitoring the situation closely. Its Radnet system had not yet detected radioactivity and it has added additional portable radiation monitors in Guam, Alaska and Hawaii.
US President Barack Obama also assured Americans that there was no danger of radiation for the country.
"I want to be very clear: We do not expect harmful levels of radiation to reach the United States, whether it's the West Coast, Hawaii, Alaska, or US territories in the Pacific," Obama said in Washington today.
"Let me repeat that: We do not expect harmful levels of radiation to reach the West Coast, Hawaii, Alaska, or US territories in the Pacific. That is the judgment of our Nuclear Regulatory Commission and many other experts."
The LA Times report said Nuclear Regulatory Commission, which oversees the US nuclear industry, "did not expect dangerous levels of radioactivity to hit the West Coast, Hawaii, Alaska or US territories in the Pacific."
The radioactive levels that reach the US could increase over time since emissions from the damaged Fukushima Daiichi nuclear power plant have grown since the disaster broke out last week, the report added.
Experts have, however, said the particles would 'wash out of the atmosphere' before they reach the US.
A specialist at nuclear watchdog group Union of Concerned Scientists Edwin Lyman said the US was not at any serious risk.
"We can never say never. My judgement is that there will probably be measurable radiation, but except for a few hot spots it is not something we should really worry about," the report quoted him as saying.
More smoke rises from Japan's crippled nuke-plant
Smoke billowed from a building at Japan's crippled nuclear power plant on Friday as emergency crews worked to reconnect electricity to cooling systems and spray more water on the overheating reactors at the tsunami-ravaged facility.
Four of the troubled Fukushima Dai-ichi plant's six reactors have seen fires, explosions or partial meltdowns in the week since the tsunami. While the reactor cores where energy is generated are a concern, Japanese and U.S. officials believe a critical danger are the pools used to store spent nuclear fuel: fuel rods in one pool were believed to be at least partially exposed and in danger of leaking radiation.
Friday's smoke came from Unit 2, and its cause was not known, the nuclear safety agency said. An explosion had hit the building on Tuesday, possibly damaging a crucial cooling chamber that sits below the reactor core.
More urgent, Japan's chief government spokesman said, was the adjacent Unit 3. Fuel rods there may have been partially exposed, and without enough water, the rods may heat further and possibly spew radiation. Frantic efforts were made Thursday to douse the unit with water, using helicopters and firetrucks, and authorities prepared to repeat the effort Friday.
"Dealing with Unit 3 is our utmost priority," Chief Cabinet Secretary Yukio Edano told reporters.
In the week since the massive earthquake and tsunami, Japan's government and the utility that runs Fukushima have struggled to contain the plant's cascading troubles.
Edano said on Friday that Tokyo is asking the U.S. government for help and the two are discussing the specifics. "We are coordinating with the U.S. government as to what the U.S. can provide and what people really need," Edano said.
The U.S. and Japan, close allies, have offered differing assessments over the dangers at Fukushima in recent days.
U.S. Nuclear Regulatory Commission Chairman Gregory Jazcko said in Washington Thursday that it could take days and "possibly weeks" to get the complex under control. He defended the U.S. decision to recommend a 50-mile (80-kilometer) evacuation zone for its citizens, wider than the 30-mile (50-kilometer) band Japan has ordered.
Crucial to the effort to regain control over the Fukushima plant is laying a new power line to the plant, allowing operators to restore cooling systems to the reactors. The operator, Tokyo Electric Power Co., missed a deadline late Thursday but said Friday workers hoped to complete the effort, first reconnecting Unit 1.
Also Friday, the Group of Seven major industrialized countries agreed to support Japan — whose infrastructure and industries were badly battered by the disasters — by intervening in currency markets. The group did not say what it would do but the efforts would likely focus on weakening the Japanese yen, which has risen this week. A strong yen could make Japanese exports less competitive, crimping any recovery.
The quake and unfolding nuclear crisis have led to power shortages in Japan, forced auto and other factories to close, sending shockwaves through global manufacturing and trade, and triggered a plunge in Japanese stock prices.
Japan disaster dead, missing toll tops 16,600: police
The number of people confirmed dead in the March 11 earthquake and tsunami in Japan has hit 6,539, surpassing the toll from the massive tremor in Kobe in 1995, police said on Friday.
The number of people unaccounted for rose slightly to 10,354, putting the combined total of dead and missing at 16,893, the National Police Agency said in its latest update. A total of 2,513 people were injured.
In January 1995, a 7.2-magnitude quake struck the western Japanese port city of Kobe, killing 6,434 people.
The March 11 quake is now Japan's deadliest natural disaster since the 1923 Great Kanto Earthquake, which killed more than 142,000 people.
The toll from the disaster one week ago has increased steadily in recent days, and reports suggest it could eventually be much higher.
The mayor of the coastal town of Ishinomaki in Miyagi prefecture said late Wednesday that the number of missing there was likely to hit 10,000, Kyodo News reported.
On Saturday, public broadcaster NHK reported that around 10,000 people were unaccounted for in the port town of Minamisanriku in the same prefecture.
Amid a mass rescue effort there were grim updates indicating severe loss of life along the battered east coast of Honshu island, where the monster waves destroyed or damaged tens of thousands of homes and other buildings.
After quake, Japan economy faces nuclear threat
Japan's earthquake-battered economy faces a new and unpredictable threat from a deepening nuclear crisis and power blackouts that analysts say could hinder a recovery from the disaster.
The 9.0-magnitude tremor and killer tsunami which struck on March 11 are feared to have caused hundreds of billions of dollars in damage to the world's third-largest economy, although the full impact is still far from clear.
The four worst affected prefectures account for about six to seven per cent of the Japanese economy, Barclays Capital analysts estimate.
The calamity has already taken a heavy toll on the industrial sector, with corporate titans such as Toyota and Sony suspending factory output.
"The economy will undoubtedly contract sharply in the near term due to shocks to both supply and demand," said Morgan Stanley economist Takehiro Sato.
But history suggests that the economic impact of natural disasters such as major quakes is usually temporary and mitigated by reconstruction efforts.
"In the near term, damage to Japan's nuclear power plants, transport system, and infrastructure will disrupt energy, water, and other production inputs, crippling activity across both the industrial and service sectors," analysts at Moody's Analytics wrote in a research note.
"The situation in Japan will improve in the third quarter, when reconstruction efforts financed by emergency public spending will help lift growth," they added.
But the fear of a major disaster at a stricken Japanese nuclear power plant which released radiation into the atmosphere after a series of explosions has taken the Japanese economy into uncharted territory.
"Given the uncertain probabilities associated with a calamitous nuclear event, the markets have responded with a push of the panic button," said David Zervos, head of global fixed income strategy at broker Jefferies and Co.
Tokyo stocks suffered the biggest two-day sell-off for 24 years on Monday and Tuesday, plunging 16 per cent, although they have since showed signs of stabilising as the central bank pumped massive funds in the financial system.
The fear is that a full-blown nuclear catastrophe would have long-lasting effects on the economy, particularly consumer sentiment and food exports.
Electricity blackouts could also lead to prolonged disruption to production of cars, flat-screen televisions and electronic gadgets key to Japanese growth.
"The big uncertainty about this disaster is that roughly 10 percent of electricity generation capacity (both nuclear and coal) may be off line for a few months, until oil- and gas-fired plants can ramp up," said Nariman Behravesh, chief economist at US consultancy firm IHS.
"In the near-term, this could have major negative ramifications for the Japanese industrial sectors," he warned.
Another major concern is the strength of the yen, which has hit a record high against the dollar - a move Tokyo has blamed on speculators betting that Japanese firms will repatriate funds from overseas to help fund reconstruction.
A stronger currency can have a severe impact on Japan's key export sector.
Damage to roads, bridges, sea and air ports as well as power plants suggests the economic damage from the quake is likely to be several tens of trillions of yen (hundreds of billion of dollars), said analysts at Morgan Stanley.
In the wake of the Great Hanshin earthquake in 1995 which levelled much of the Japanese port city of Kobe, industrial output quickly bounced back after an initial slump, while massive stimulus spending helped to support the economy.
The latest disaster comes with Japan's economy already in the doldrums, struggling to recover from the 1990s "lost decade" and the global economic slump sparked by the 2007-2008 crisis.
The initial shock of the latest disaster "might pull the economy back into recession territory," warned analysts at Standard and Poor's.
Japan's economy contracted by an annualised 1.3 per cent in the final quarter of 2010 and Credit Agricole economist Susumu Kato said growth was now likely to shrink for three straight quarters, falling into a "temporal recession."
"As consumers become even more careful and increase precautionary saving for rainy days, that will sharply weigh on private consumption in those quarters," Kato said.
But reconstruction activities from the July-September quarter will result in a return to positive growth thereafter, he added.
Repairing the damage inflicted by the earthquake is expected to set back Japan's efforts to reduce what is the industrialised world's biggest debt, at around 200 per cent of gross domestic product, and may require tax increases.