Japan's nuclear safety agency says an explosion has been heard at Unit 2 of the Fukushima Dai-ichi nuclear plant.
An agency spokesman speaking Tuesday on national television said the explosion was heard at 6:10 a.m. (2110 GMT).
No other details were immediately announced.
The troubles at the Fukushima Dai-ichi complex began when Friday's massive earthquake and tsunami in Japan's northeast knocked out power, crippling cooling systems needed to keep nuclear fuel from melting down.
