TOKYO - (AP) - Japan's tsunami-stricken nuclear power plant wasleaking highly radioactive water into the sea Saturday, nuclearsafety officials said.

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

The water was seeping Saturday from a newly discovered crack ina maintenance pit on the edge of the Fukushima Dai-ichi nuclearsite into the Pacific Ocean, Japan Nuclear and Industrial SafetyAgency spokesman Hidehiko Nishiyama said.

Measurements show the air right above it contained 1,000millisieverts of radioactivity. Exposure to 500 millisieverts overa short period of time can increase the long-term risk of cancer.But experts say radiation is quickly diluted by the vast Pacificand that even large amounts have little effect.

It wasn't immediately clear whether workers who have beenrushing to bring the reactors under control were exposed. Peopleliving within 12 miles (20 kilometers) of the Fukushima plant havebeen evacuated.

Nishiyama said officials will check the level of radiation inseawater near the reactor as well as seawater around 9 miles (15kilometers) from the reactor. They will use concrete to seal the8-inch (20-centimeter) crack and try to stop the radiation fromleaking.

"This could be one of the sources of seawater contamination,"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 hasbeen detected in seawater just off the plant.