The operator of Japan's quake damaged Fukushima No. 1 nuclear power plant on Monday confirmed that radioactive water from the plant may have leaked into the ocean, sparking fresh concerns about the nation's ongoing nuclear crisis.

Asia's largest utility firm Tokyo Electric Power Company, also known as Tepco, said that around 45 tons of water contaminated with radioactive substances caesium and iodine had likely run into the ocean from a gutter system into which water had leaked from a condensation unit.

Junichi Matsumoto, a general manager at Tepco, said the leaked water contained 16,000 becquerels per liter of radioactive cesium 134 and 29,000 becquerels of cesium 137, surpassing government safety limits by 267 and 322 times respectively.

Local media reports have also said that the contaminated water may also contain other radioactive substances such as strontium, known to cause bone cancer in humans.

Tepco officials told the press on Monday that contaminated water leaked from a desalination unit through a crack in a concrete wall into a gutter. The gutter connects with a drain that flows freely into the Pacific Ocean, the utility said.

The remaining contaminated water that leaked from the desalination unit was being pumped from the building, Tepco added.

The utility also said that sand bags were being used as an emergency means to prevent further leakage from the unit, but that it could take up to three weeks to know the exact amount of leaked radioactive water.

"We will keep investigating the problem with the system," a spokesperson for the utility said. Tepco admitted that after a series of previous leaks they had not done enough to contain the problem and had wrongly assumed the water would not leak into drainage systems.

Despite Tepco's latest oversight, the utility said the radioactive leak would not affect plans to bring the plant to a state known as cold shutdown by the end of this year, in a step towards decommissioning the facility entirely.

Separately, the Japanese government on Monday halted shipments of rice from some farms in Fukushima prefecture due to the yields containing high levels of radioactive cesium.

Rice crops containing radioactive cesium reaching levels between 510 and 590 becquerels per kilogram were found at three farms in the district and the latest ban marks the third embargo on rice shipments from the prefecture following the March nuclear disaster there.