Radioactive water is reported to be leaking into the sea from a 30cm crack in a containment pit at Japanâ€™s quake-hit Fukushima nuclear plant.
The crack, under reactor 2, is believed to be the source of recent radiation in coastal waters.
Japanese Deputy Director-general of the Nuclear and Industrial Safety Agency, Hidehiko Nishiyama, told journalists that plans are underway to pour concrete into the pit to seal the crack.
A massive search has begun to find the remains of those missing since the devastating tsunami hit.
More than 11 500 people are confirmed dead, but nearly 16 500 remain unaccounted for.
Many coastal areas remain inaccessible to rescuers trying to enter by road or foot, blocked by the mangled remains of houses, ships, cars and trains.