Japan raised the severity of its nuclear disaster to the highest level on Tuesday, putting it on a par with the world's worst disaster nuclear accident at Chernobyl after another major aftershock rattled the quake-ravaged east.

Engineers were no closer to restoring the cooling systems at the plant's reactors, critical to bringing down the temperature of overheated nuclear fuel rods, showing that the battle to contain the damage was far from over, although a fire at the plant appeared to have been extinguished.

The rating of the accident at the Fukushima Daiichi nuclear plant was raised to 7, the worst on an internationally recognised scale, from a 5-rating. Japan said this reflects the initial severity of the crisis and not the current situation.

'This is a preliminary assessment, and is subject to finalisation by the International Atomic Energy Agency,' said an official at the Nuclear and Industrial Safety Agency (NISA), the government's nuclear watchdog, which made the announcement with the Nuclear Safety Commission.

Nuclear industry specialist Murray Jennex, an associate professor at San Diego State University in California, dismissed the comparison.

'It's nowhere near that level. Chernobyl was terrible - it blew and they had no containment, and they were stuck. Their (Japan's) containment has been holding, the only thing that hasn't is the fuel pool that caught fire,' he said.

A level 7 incident entails a major release of radiation with widespread health and environmental effects, while a 5-rated event, which is what Fukushima is currently classified as, is a limited release of radioactive material, with several deaths from radiation, according to the International Atomic Energy Agency.

The scale is designed so the severity of an event is about 10 times greater for each increase in level. The 1979 US nuclear accident at Three Mile Island was a 5-rated incident.

More aftershocks

News of the fire at the plant came only minutes after a 6.3 aftershock struck off the coast of Chiba, 77 km (48 miles) northwest of Tokyo. Kyodo said Japan's main international airport Narita closed runways for checks but later resumed flights.

An aftershock measuring 6.6 quake hit Fukushima prefecture on Monday evening, temporarily cutting power and forcing workers to evacuate the nuclear plant.

Japan's nuclear agency NISA said Monday's aftershock, which killed one and knocked out power to 220,000 households, did not damage the plant.

There have been hundreds of aftershocks since March 11, when a massive 9 magnitude earthquake and 15 metre tsunami hit northeast Japan, plunging the country into its worst crisis since World War Two.

Nearly 28,000 Japanese are dead or missing and the world's third-largest economy is reeling with power blackouts, factory  closures and cuts to supply lines. The disaster is estimated to cost heavily indebted Japan some $300 billion, making it the world's most costly.

The country's nuclear commission also released a preliminary calculation for the cumulative amount of external exposure to radiation, saying it had exceeded the yearly limit of 1 millisieverts in areas extending more than 60 kms (36 miles) to the northwest of the plant and about 40 km to the south-southwest, Kyodo reported.

TEPCO said on Monday it had stopped the discharge of low-level radioactive water into the sea that have drawn complaints from neighbouring China and South Korea.

It has already pumped 10,400 tonnes of low-level radioactive water into the ocean to free up storage capacity for highly contaminated water from the reactors.

In a desperate move to cool the highly radioactive fuel rods, Tepco has pumped water onto reactors, some of which have experienced a partial meltdown.

But the strategy has hindered moves to restore the plant's internal cooling system as engineers have had to focus on how to store 60,000 tonnes of contaminated water.

Engineers are also pumping nitrogen into reactors to counter a build-up of hydrogen and prevent another explosion sending more radiation into the air, but they say the risk of such a dramatic event has lowered significantly since March 11.

Because of accumulated radiation contamination, the government is encouraging people to leave certain areas beyond its 20 km (12 mile) exclusion zone around the plant. Thousands of people could be affected by the move. – Reuters