Radioactive water is leaking into the sea from a 30cm crack in a containment pit at Japan's quake-hit Fukushima nuclear plant.
The Tokyo Electric Power Company says the crack under reactor 2 may be the source of recent radiation in coastal waters.
TEPCO is preparing to pour concrete into the pit to try to stop the leak reports the BBC.
Prime Minister Naoto Kan has been visiting the area worst affected by the tsunami, arriving in a town where more than 2000 are listed as dead or missing.
He flew into Rikuzentakata on a military helicopter from Tokyo, stopping over in nearby Fukushima prefecture, in a show of support for emergency crews risking their lives to prevent meltdown at a nuclear plant.
In Rikuzentakata, Iwate prefecture, 400km northeast of Tokyo, around 1000 people are confirmed dead and 1300 are still missing as rescue work enters its fourth week after the 11 March disaster.
Mr Kan has warned of a long-term battle at Fukushima.
Officials say the evacuation of residents near the nuclear plant will be long-term.
Many people evacuated from the area around the plant are living in temporary shelters.
Meanwhile, in the northeast of the country, thousands of military personnel are involved in an intensive search for the remains of victims from last month's earthquake and tsunami that left 28,000 people dead or missing.
