Japan's quake damaged nuclear power plant leaked about 45 tons of highly radioactive water and some may have drained into the Pacific Ocean.

The operator of Japan's crippled Fukushima No. 1 nuclear power plant confirmed it on Monday, sparking fresh concerns about the country's ongoing nuclear crisis.

The Tokyo Electric Power Company (Tepco), Asia's largest utility firm, said the water contaminated with radioactive substances caesium and iodine came from a gutter system into which water had leaked from a condensation unit.

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

A pool of radioactive water was discovered midday Sunday around a decontamination device and the leak appeared to stop after the equipment was turned off. Later, workers found a crack in a concrete barrier leaking the water into the gutter, China Daily reported via agencies.

Tepco 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 spokesman of the firm said.

The firm 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.

The pooled water around the purification device was measured Sunday at 16,000 bequerels per liter of cesium-134, and 29,000 bequerels per liter of cesium-137, TEPCO said. That's 270 times and 322 times higher than government safety limits respectively, according to the Citizens' Nuclear Information Center in Tokyo.

Cesium-137 is dangerous because it can last for decades in the environment, releasing cancer-causing radiation. The half-life of cesium-134 is about two years, while the half-life of cesium-137 is about 30 years, China Daily reported.

TEPCO is using the purification devices to decontaminate water that has been cooling the reactors. Three of the plant's reactor cores mostly melted down when the March 11 tsunami knocked out the plant's cooling system.

The leak is a reminder of the difficulties facing Tepco as it tries to meet its goal of bringing the tsunami-damaged Fukushima Dai-ichi nuclear power plant to a cold shutdown by year's end, the report said.

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.