TOKYO -- Japan has decided to raise the severity level of the accident at the crippled Fukushima Daiichi nuclear plant to 7, the worst on an international scale, from the current 5, Kyodo news reported on Tuesday, citing government sources as saying.

The report came as the government expanded an evacuation zone around the crippled Fukushima nuclear plant because of the high levels of accumulated radiation since a 15-metre tsunami ripped through the complex a month ago, causing massive damage to its reactors which engineers are still struggling to control.

The Kyodo report said that the high levels of radiation that have been released by the Fukushima Daiichi plant meant it could raise the severity level from 5 to the highest 7, the same as the 1986 Chernobyl accident.

It said the government's Nuclear Safety Commission had estimated that at one stage the amount of radioactive material released from the reactors in northern Japan had reached 10,000 terabequerels per hour of radioactive iodine 131 for several hours, which would classify the incident as a major accident according to the INES scale.

The International Nuclear and Radiological Event Scale (INES) is published by the International Atomic Energy Agency and ranks nuclear and radiological accidents and incidents by severity from 1 to a maximum of 7.

The Kyodo report did not say when the estimate related to.

Japan had previously assessed the accident at reactors operated by Tokyo Electric Power Co (TEPCO) at level 5, the same level as the Three Mile Island accident in the United States in 1979.

The tsunami was triggered by March 11 9.0 magnitude earthquake, the largest recorded in quake-prone Japan, crippling the reactors' cooling systems.

A spokesman for the Nuclear and Industrial Safety Agency, Japan's nuclear safety watchdog, said on Tuesday that the level of the Fukushima incident was still a 5 and that he was unaware of any move by the government to raise the level.

TEPCO said it had stopped the discharge of low-level radioactive water into the sea that had 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.

On Monday, shortly after Japan marked one month since the quake, a huge aftershock shook a wide swathe of eastern Japan, killing two people, and knocking out power to 220,000 homes.

It was one of more than 400 aftershocks above a 5 magnitude to have hit the area 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.

"These new evacuation plans are meant to ensure safety against risks of living there for half a year or one year," he said. There was no need to evacuate immediately, he added.

TEPCO President Masataka Shimizu visited the area on Monday for the first time the disaster. He had all but vanished from public view apart from a brief apology shortly after the crisis began and has spent some of the time since in hospital.

"I would like to deeply apologise again for causing physical and psychological hardships to people of Fukushima prefecture and near the nuclear plant," said a grim-faced Shimizu.

Dressed in a blue work jacket, he bowed his head for a moment of silence with other TEPCO officials at 2:46 pm (0546 GMT), exactly a calendar month after the earthquake hit.

RADIOACTIVE WATER

Engineers at the plant north of Tokyo said they were no closer to restoring the plant's cooling system, which is critical to bring down the temperature of overheated fuel rods and to bringing the six reactors under control.

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

The triple disaster is the worst to hit Japan since World War Two, leaving nearly 28,000 dead or missing and rocking the world's third-largest economy.

Concern at the government's struggle to handle the situation is mounting, with Prime Minister Naoto Kan's ruling party suffering embarrassing losses in local elections on Sunday.

Voters vented their anger at the government's handling of the nuclear and humanitarian crisis, with Kan's ruling Democratic Party of Japan losing nearly 70 seats in local elections.

($1=85.475 Japanese yen)