Japan's nuclear regulator says it "strongly suspects" that highly radioactive groundwater at the tsunami-hit Fukushima nuclear power plant is leaking into the adjoining Pacific Ocean.

"We must find the cause of the contamination ... and put the highest priority on implementing countermeasures," Shunichi Tanaka, chairman of the Nuclear Regulation Authority, told the Kyodo news agency Wednesday.

Tanaka made the remarks after levels of radioactive cesium and strontium detected in groundwater samples rose sharply in recent days.

The main challenge in dismantling the plant, which suffered major damage in a magnitude-9.0 earthquake and subsequent tsunami on March 11, 2011, is the accumulation of contaminated water in the soil around buildings housing the crippled nuclear reactors.

The amount of radioactive liquid is increasing daily due to leaks from the plant's refrigeration system and the inflow of groundwater from nearby areas.

Although the cause of the sharp increase in levels of radioactive particles in the groundwater remains unclear, the plant's operator, Tokyo Electric Power, said it may be leaking from a well adjacent to the ocean.

But the NRA said it doubted that was the only source of the contamination.

Radiation levels from groundwater samples analyzed Tuesday by TEPCO were 100 times higher than those found in tests conducted just four days earlier.

The sample contained 11,000 becquerels of cesium-134 per liter and 22,000 becquerels of cesium-137 per liter.

TEPCO has adopted measures to seal off that contaminated water in certain areas of the plant, although the nuclear regulator says the company cannot prevent all the contaminated material from leaking into the sea or the soil.

The plant's operator said that for the moment it had not detected "a significant impact" on the environment. EFE