Japanese workers struggling to contain a crisis at a crippled nuclear plant discovered a crack in a pit leaking highly radioactive water straight into the sea, the firm operating the facility said.

In a discovery regulators said might explain the radioactive water that has hobbled efforts to quell Japan's nuclear crisis, Tokyo Electric Power Co (TEPCO) said the radiation in the pit at its No.2 reactor in Fukushima measured 1,000 millisieverts per hour.

"With radiation levels rising in the seawater near the plant, we have been trying to confirm the reason why, and in that context, this could be one source," said Hidehiko Nishiyama, deputy head of the Nuclear and Industrial Safety Agency.

He cautioned, however: "We can't really say for certain until we've studied the results."

TEPCO plans to fill in the crack, while checking radiation levels in sea water at three locations 15 kilometres offshore in addition to four spots where they have already taken samples, he said.

Since the six-reactor Fukushima plant's cooling systems were knocked out by the March 11 quake and tsunami, the danger of radioactive leaks into the environment has been the top concern.

TEPCO said earlier that radioactive iodine-131 in groundwater below Japan's stricken nuclear plant was significantly higher than the safe level set by the government.

The Nuclear and Industrial Safety Agency (NISA) confirmed that a crack had been found, saying TEPCO was trying to get a clearer picture of how the leak originated.

"TEPCO is checking where the water accumulating in the pit came from," said NISA official Hidehiko Nishiyama.

TEPCO said it was also investigating if other cracks remained in the facility causing leaks into the environment.

After three weeks, operators of the plant are no closer to regaining control of damaged reactors and fuel rods remain overheated.

Japan is facing a damages bill which may top $US300 billion - the world's biggest from a natural disaster.

Radiation 4,000 times the legal limit has been detected in seawater near the Daiichi plant and a floating tanker was to be towed to Fukushima to store contaminated seawater. But until the plant's internal cooling system is reconnected radiation will flow from the plant.

"We are trying to employ as many measures as possible [to put the plant under control]. We are holding high hopes [for this storage]," said a TEPCO official.

In its attempt to bring the plant under control, TEPCO is looking for "jumpers" - workers who, for payment of up to $US5,000 a shift, will rush into highly radioactive areas to do a quick task before racing out as quickly as possible.

"My company offered me 200,000 yen ($US2,500) per day," one subcontractor, unidentified but in his 30s, told Japan's Weekly Post magazine. "Ordinarily I'd consider that a dream job, but my wife was in tears and stopped me, so I declined."

TEPCO was also spraying resin onto radioactive dust in an attempt to stop it from being carried in the wind.

"We sprayed 2,000 litres over 500 square metres of land. We plan to evaluate the result of the test spraying on April 2nd and 3rd. It takes about 24 hours for this scattering-prevention solution to get dried," said the official.

It could take years, possibly decades, to make safe the area around the plant, 240km north of Tokyo.

With its president, Masataka Shimizu, in hospital, an enormous compensation bill looming and mounting criticism of both its handling of the crisis and prior safety preparations, TEPCO may need state help, according to media reports.

'Absolutely cannot lose'

The news comes as prime minister Naoto Kan made his first visit to the country's tsunami-devastated region and met crews who have struggled to stabilise the plant.

Donning a blue workman's outfit, Mr Kan arrived by military helicopter to give a pep talk to the atomic plant workers, firefighters and troops.

"I want you to fight with the conviction that you absolutely cannot lose this battle," he told them at their base, the J-Village football academy 20 kilometres from the plant.

He urged military personnel there to "fight to determine Japan's fate".

Hundreds of thousands remain homeless, sheltering in evacuation centres, as the death toll from the disaster rises.

Thousands of Japanese and US soldiers on Saturday conducted a search for bodies using dozens of ships and helicopters to sweep across land still under water along the north-east coast. The teams hope when a large spring tide recedes it will make it easier to spot bodies.

Mr Kan spoke with refugees living in a makeshift camp in the fishing village of Rikuzentakata, levelled by the tsunamis and massive earthquake, which left 28,000 dead and missing.

"It will be kind of a long battle, but the government will be working hard together with you until the end. I want everyone to do their best, too," Mr Kan told one survivor in a school that was now an evacuation shelter.

Despite its tsunami seawalls, Rikuzentaka was flattened into a wasteland of mud and debris and most of its 23,000 residents killed or injured, many swept away by the waves.

"A person that used to have a house near the coast told me 'Where am I supposed to build a house after this?', so I encouraged this person and said the government will provide support until the end," Mr Kan told reporters.

Unpopular and under pressure to quit or call a snap poll before the disaster, Mr Kan has been criticised for his management of Japan's humanitarian and nuclear crisis and his leadership remains in question.

"There are some evacuation centres that lack electricity and water. There are people who can't even go look for the dead. I want him to pay attention to them," said Kazuo Sato, a 45-year-old fisherman.

- AFP/Reuters