The operator of the crippled Fukushima Daiichi nuclear power plant, Tokyo Electric Power Co (TEPCO), says some eight tons of radioactive water has leaked from a reactor at the plant.
TEPCO said there was no cause for alarm, since none of the water reached outside the reactor building.
TEPCO said freezing temperatures may have caused a pipe to burst in the building.
The huge earthquake and tsunami that followed caused Japan's biggest civilian nuclear disaster in March.
Compiled from Reuters reports