Highly radioactive water was leaking into the sea Saturday from a crack discovered at a Japanese nuclear power plant destabilized by last month's earthquake and tsunami, Japanese nuclear safety officials say.

The contaminated water will quickly dissipate into the sea and is not expected to cause any health hazard, they say. Nevertheless, the disturbing discovery points at the unexpected problems that have cropped up and continue to hamper technicians trying to control the crisis at the Fukushima Daiichi nuclear plant about 220 kilometres north of Tokyo.

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A Tokyo Electric Power Co. employee points at a maintenance pit at the Fukushima Daiichi plant which has a crack inside (not visible in the photo.) (TEPCO/Associated Press )

The government had rushed to provide relief to the more than 165,000 people still living in shelters — and tens of thousands more who lack electricity or running water — after dozens of villages, towns and cities were slammed March 11 by a tsunami that followed a magnitude 9.0 earthquake.

But its attention has been divided by the efforts to stabilize the Fukushima Daiichi plant, which suffered heavy damage and has dragged the country to its worst nuclear crisis since the atomic bombings of Hiroshima and Nagasaki during the Second World War.

The plant's reactors overheated to dangerous levels after electrical pumps, deprived of electricity, failed to circulate water to keep the reactors cool. A series of almost daily problems have led to substantial amount of radiation leaking in the atmosphere, ground and sea.

On Saturday, workers discovered a 20-centimetre long crack in a maintenance pit that was leaking highly radioactive water into the Pacific Ocean, said Japan Nuclear and Industrial Safety Agency spokesman Hidehiko Nishiyama.

He said the water, contaminated with radioactive iodine far above the legal limit found inside the pit, could be one of the sources of recent spikes in radioactivity in seawater.

"There could be other similar cracks in the area, and we must find them as quickly as possible," he told reporters.

Soon after the discovery, the plant's operator, Tokyo Electric Power Co., started filling the pit with cement to seal the crack and prevent more contaminated water from seeping into the ocean.

Nuclear safety officials said the crack was likely caused by the quake and may be the source of radioactive iodine that started showing up in the ocean more than a week ago.

People living within 20 kilometres of the plant have been evacuated from the area, and the radioactive water will quickly dissipate in the sea, but it was unclear if the leak posed any new danger to workers.

The cracked pit houses cables for one of the six nuclear reactors, and the concentration of radioactive iodine was the same as in a puddle of contaminated water found outside the reactor earlier in the week. Because of that, officials believe the contaminated water is coming from the same place, though they are not sure where.

PM's visit fails to reassure

Prime Minister Naoto Kan toured the town of Rikuzentakata on Saturday, his first trip to survey damage from the tsunami, but his visit did little to alleviate the radiation worries compounding the misery for people trying to recover from the disaster. 

"The government has been too focused on the Fukushima power plant rather than the tsunami victims. Both deserve attention," said 35-year-old Megumi Shimanuki, who was visiting her family at a community centre converted into a shelter in hard-hit Natori, about 160 kilometres from Rikuzentakata.  

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Japanese Prime Minister Naoto Kan, right, speaks to an employee of Tokyo Electric Power Co. during a visit to J-Village, a national training centre in the Fukushima prefecture of Naraha. (Japanese Prime Minister's Office/Associated Press)

The double disaster is believed to have left 25,000 people dead. Just under 12,000 people have been confirmed dead.

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

He earlier met the mayor, whose 38-year-old wife was swept away. Kan bowed his head for a moment of silence in front of the town hall, one of the few buildings still standing, though its windows were blown out and metal and debris sit tangled out front.

He also stopped at the sports complex being used as a base camp for nuclear plant workers, who have been hailed as heroes for labouring in dangerous conditions. He had visited the nuclear crisis zone once before, soon after the quake.

Radiation is also a concern for people living around the plant. In the city of Koriyama, Tadashi and Ritsuko Yanai and their one-month-old baby have spent the past three weeks in a sports arena converted into a shelter.

Baby Kaon, born a week before the quake, has grown accustomed to life there, including frequent radiation screenings, but his parents have not. Their home is fine, but they had to leave because it is 10 kilometres from the nuclear plant.

Asked if he had anything he would like to say to the prime minister, Tadashi, 32, paused to think and then replied: "We want to go home. That's all, we just want to go home."

In Natori, where about 1,700 people are living in shelters, others had stronger words for Kan. Toru Sato, 57, lost both his wife and his house in the tsunami and said he was bothered that Kan's visit to the quake zone was so brief, about a half day.

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