An aide to Prime Minister Naoto Kan says the government's main aim is to stop radiation leaks from the stricken Fukushima Daiichi nuclear plant that are spooking Japanese, hindering work and frightening tourists.

Plant operator Tokyo Electric Company (TEPCO) has poured concrete into a crack in a concrete pit in reactor no. 2 but this has failed to stop leaks.

TEPCO is also using water-absorbent polymers, with plans to top the polymers with more concrete to halt the leaks.

Engineers examine alternatives to pumping in water to cool the reactor, including an improvised air conditioning system, spraying fuel rods with vaporised water or using the plant's cleaning system.

Radiation levels in the sea nearby stand at 4,000 times the legal limit.

A group of farmers from the Fukushima region came to Tokyo with Geiger counters to show their produce is safe.

Prime Minister Naoto Kan ensured residents in the disaster zone on Saturday that the government would offer support in rebuilding. He entered the 20-km evacuation zone around the wrecked plant and visited "J-village", a sports facility serving as headquarters for emergency teams tiring to cool down reactors.

Kan said on Friday that TEPCO should remain in private hands, even though the company would need financial assistance from the government to deal with the aftermath of the disaster.

Earlier, the Manichi newspaper said the government would take control of TEPCO.

Chief Cabinet Secretary Yukio Edano said the evacuation of residents near the plant will be a "long-term" operation.

A UN watchdog on Thursday suggested widening of the exclusion zone around the station after radiation measured at a village 40 km distant exceeded a criterion for evacuation.

Japanese manufacturing activity slumped to a two-year low in March and posted the sharpest monthly fall on record as the quake and tsunami hit supply chains and output.

A total of 11,938 people are confirmed dead by Japan's National Police Agency, while 15,478 are missing.

A total of 168,586 households are without electricity and 220,000 without running water.

More than 164,200 people are living in shelters on high ground above vast plains of mud-covered debris.

Thousands of Japanese and US Soldiers conducted a search for bodies on Saturday using dozens of ships and helicopters to sweep across land still under water along the northeast coast.

The cost of damage is estimated around $300 billion, making it the world's costliest natural disaster.

The 1995 Kobe quake cost $100 billion while Hurricane Katrina in 2005 caused $81 billion in damage.