Japan's nuclear regulator has officially raised the severity rating of the latest radioactive water leak at the crippled Fukushima nuclear plant.

It says the crisis is now at Level 3, or a "serious incident", up from a Level 1 "anomaly", in consultations with the International Atomic Energy Agency.

The plant's operator, Tokyo Electric Power Company, said last week that 300 tonnes of highly radioactive water had leaked from a storage tank at the facility.

The utility still does not know how long the water may have been leaking, and says it was possible the contaminated water may have reached the Pacific Ocean.

Originally the leak was caused by reactor meltdowns at the plant in March 2011, which were triggered by a massive earthquake and tsunami.