Nuclear safety spokesman Hidehiko Nishiyama says the air above the leak contains 1,000 millisiverts of radioactivity. Exposure to 500 millisiverts over a short period of time can increase the risk of cancer.

The water was seeping Saturday from a crack in the containment for a maintenance pit on the edge of the nuclear site.

The Fukushima Dai-ichi plant has been spewing radioactivity since it was hit by a tsunami three weeks ago.