Japan's tsunami-stricken nuclear power plant is leaking highly radioactive water into the sea, nuclear safety officials said.

The plant has been spewing radioactivity since March 11, when a magnitude-9.0 earthquake and ensuing tsunami knocked out power, disabling cooling systems and allowing radiation to seep out of the overheating reactors.

The water was today seeping from a newly discovered crack in a maintenance pit on the edge of the Fukushima Dai-ichi nuclear site into the Pacific Ocean, Japan Nuclear and Industrial Safety Agency spokesman Hidehiko Nishiyama said.

Measurements show the air immediately above it contained 1,000 millisieverts of radioactivity.

Exposure to 500 millisieverts over a short period of time can increase the long-term risk of cancer. But experts say radiation is quickly diluted by the vast Pacific and that even large amounts have little effect.

It is not clear whether workers who have been rushing to bring the reactors under control have been exposed. People living within 12 miles (20 kilometres) of the Fukushima plant have been evacuated.

Mr Nishiyama said officials will check the level of radiation in sea water near the reactor and about 9 miles (15 kilometres) away from it. They will use concrete to seal the 8-inch (20-centimetre) crack and try to stop the radiation from leaking.

"This could be one of the sources of seawater contamination," Mr Nishiyama said. "There could be other similar cracks in the area, and we must find them as quickly as possible."

Over the past week, radioactivity beyond the legal limit has been detected in seawater just off the plant.