The operator of Japan's quake-stricken nuclear plant said on Saturday it had found radioactive water leaking into the sea from a cracked concrete pit at its No 2 reactor in Fukushima.

Tokyo Electric Power Co (TEPCO) officials said it was possible that the crack may be the source of recent radiation contamination in the sea off the northeast coast.

The radiation in the pit measured 1,000 millisieverts per hour. It was not clear whether the water had come from the reactor itself nor how much has been leaking from the pit into the sea.

Hidehiko Nishiyama, deputy director-general of the Nuclear and Industrial Safety Agency, told a separate news conference that TEPCO was planning to pour concrete into the pit to seal the crack.

"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. We're testing samples of water from the pit and from the sea near the plant, and we can't really say for certain until we've studied the results," he said. ? (Reporting by Chisa Fujioka and Chizu Nomiyama, editing by Jonathan Thatcher)