Japanese nuclear safety officials said the crippled Fukushima power plant was leaking highly radioactive water into the sea on Saturday, as the prime minister surveyed the villages damaged by a massive tsunami several weeks ago.

The Fukushima Dai-ichi facility has been spewing radiation across the country's northeast coast since March 11, when a massive earthquake and tsunami knocked out power and disabled cooling systems, overheating several reactors.

Nuclear safety officials said they recently discovered a crack in a maintenance pit on the edge of the complex, which may have been leaking radioactive water into the Pacific Ocean since the earthquake.

Experts say radiation quickly disperses in both air and water and expect the radioactive water would be quickly diluted in the vast ocean.

TEPCO was attempting to use concrete to seal the 20-centimetre crack on Saturday.

"This could be one of the sources of seawater contamination," Nuclear and Industrial Safety Agency spokesman Hidehiko Nishiyama said. "There could be other similar cracks in the area, and we must find them as quickly as possible."

Residents within 20 kilometres of the plant were evacuated two weeks ago, while those up to 30 kilometres away have been urged to leave voluntarily or to stay indoors. But sporadic readings of higher-than-recommended radiation have been detected within a 40-kilometre radius.

Radiation problems have added to the suffering of those who are trying to cope in the disaster's wake. Nearly 12,000 people are confirmed dead, although as many as 25,000 people may have died in the earthquake and tsunami.

On Saturday, Prime Minister Naoto Kan surveyed the damaged coastal area, where dozens of villages and cities were left pulverized by the tsunami.

In Rikuzentakata, a town left with few standing buildings, Kan met with the mayor, whose wife had been swept away by the wave and remains missing, and offered a prayer to the residents.

"The government fully supports you until the end," Kan later told the 250 evacuees at an elementary school, which is serving as an evacuation centre.

But some locals told the Associated Press that Kan didn't spend enough time with survivors in the wake of the tsunami, instead focusing on the Fukushima power plant disaster.

"Both deserve attention," said Megumi Shimanuki, 35, who was visiting family members at a shelter in Natori, about 160 kilometres from the area Kan travelled to.

Others, like Toru Sato, had stronger words for the prime minister. The 57-year-old, who lost his wife and his house in the tsunami, said he was upset that Kan spent only about a half day in the area.

"He's just showing up for an appearance," Sato told The Associated Press. "He should spend time to talk to various people, and listen to what they need."

The prime minister's visit came as Japan's nuclear safety agency orders a review of the latest radiation measurements from around the crippled Fukushima plant, saying they seemed suspiciously high.

Radiation concerns have rattled the Japanese public as it struggles to return to normal life. Three weeks after the earthquake-generated tsunami, more than 165,000 people are living in shelters. Some 60,000 other families are still living without running water and another 170,000 do not have electricity.

It is also becoming clear that some 70,000 local residents already evacuated from around the damaged nuclear complex may not be able to return for some time yet.

Japan's Chief Cabinet Secretary Yukio Edano said the evacuation would be a long-term operation. Some nuclear experts say it could take years -- possibly decades -- to make the area around the plant safe.

The event looks likely to be the world's costliest natural disaster, with estimates of damage topping US$300 billion.

With files from The Associated Press