Japan on Monday expanded the evacuation zone around its crippled nuclear plant because of high levels of accumulated radiation in the area a month after an earthquake and tsunami sparked the worst nuclear crisis since Chernobyl.

Chief Cabinet Secretary Yukio Edano said villages and towns outside the 20 km (12 mile) evacuation zone that have had more accumulated radiation would be evacuated.

Children, pregnant women, and hospitalised patients should stay out of some areas 20-30 km from the Fukushima nuclear complex, he added.

"We have made a new decision about evacuations based on data analysis of accumulated radiation exposure information," Edano told a news conference.

"There is no need to evacuate immediately," he added, but said it would be desirable to proceed with the new evacuation over a one-month period.

Japan had steadfastly refused to extend the zone despite international concerns over radiation spreading from the six damaged reactors at Fukushima which engineers are still struggling to bring under control after they were wrecked by the 15-metre tsunami.

Residents of one village, Iitate which is 40 km from the Fukushima Daiichi plant, have been told to prepare for evacuation because of prolonged exposure to radiation, a local official told Reuters by phone. It has a population of 5,000.

The International Atomic Energy Agency has urged Japan to extend the zone and some countries, including the United States, have advised their citizens to stay 80 km away from the plant.

Greenpeace International has called for full evacuation of the 30 km zone because "radiation does not spread in circles," said Jan Van de Putte, a safety advisor to the group. He said children and pregnant women should be evacuated first.

The president of Tokyo Electric Power Co (TEPCO) , which operates the plant, visited the area on Monday for the first time the March 11 disaster.

Masataka Shimizu had all but disappeared 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 p.m. (0546 GMT), exactly a month after the earthquake hit.

Fukushima Governor Yuhei Sato refused to meet Shimizu during his visit, but the TEPCO boss left a business card at the government office.

Sato has criticised the evacuation policy, saying residents in a 20-30 km radius were initially told to stay indoors and then advised to evacuate voluntarily.

Engineers at the damaged Daiichi plant north of Tokyo said they were no closer to restoring the plant's cooling system which is critical if overheated fuel rods are to be cooled and the six reactors brought under control.

In a desperate move to cool highly radioactive fuel rods, operator Tokyo Electric Power Co (TEPCO) <9501.t> 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, critical to end the crisis, as engineers have had to focus how to store 60,000 tonnes of contaminated water.

Engineers have been forced to pump low-level radioactive water, left by the tsunami, back into the sea in order to free up storage capacity for highly contaminated water from reactors.

China and South Korea have both criticised Japan for pumping radioactive water into the sea, with Seoul calling it incompetent, reflecting growing international unease over the month-long atomic disaster and the spread of radiation.

TEPCO hopes to stop pumping radioactive water into the ocean on Monday, days later than planned.

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 after a 9.0 magnitude earthquake and a huge tsunami battered its northeast coast, leaving nearly 28,000 dead or missing and rocking the world's third-largest economy.

Concern at Japan's inability contain its nuclear crisis is mounting with Prime Minister 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.

The unpopular Kan was already under pressure to step down before March 11, but analysts say he is unlikely to be forced out during the crisis, set to drag on for months.

"The great disaster was a double tragedy for Japan. The first tragedy was the catastrophe caused by the earthquake, tsunami and the nuclear accident. The other misfortune was that the disaster resulted in prolonging Prime Minister Kan's time in office," Sankei newspaper said in an editorial on Monday.