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Contaminated water has leaked from a treatment system at Japan's crippled Fukushima nuclear plant, with some potentially entering the sea, the plant's operator says.

Workers found the leak - caused by a crack in a concrete foundation - on Sunday morning.

Tepco said some 45 cubic metres (1,590 cubic feet) of water had escaped before sandbags blocked the leak.

Some water leaked into a drainage ditch that flowed out to sea, it added.

The nuclear plant was crippled by the 11 March earthquake and tsunami. Blasts occurred at four of the six reactors at the plant after key cooling systems were knocked out.

Work to bring the plant to a cold shutdown by the end of the year is continuing, and a 20km (12m) exclusion zone remains in place.

The leak occurred in a facility that removes radioactive caesium from seawater used to cool the reactors.

"We are currently assessing the situation and based on the amount of water which has leaked into the ocean there may or may not be an effect on the environment," Tepco spokesman Junichi Matsumoto said.

Tepco - Tokyo Electric Power Company - estimated about 300 litres of water (528 pints) had flowed into the drain before the leak was stopped.

It said the water contained caesium at levels ""roughly the same as or slightly higher" than seawater near the plant, and may also have contained strontium, which can cause bone cancer.

It said it would take several days to determine how much strontium was in the water.