Japan's nuclear regulator says it believes radioactive water from the crippled Fukushima power station is leaking into the Pacific, and urged operators to act to prevent further contamination.

Shunichi Tanaka, head of the new Nuclear Regulation Authority, said on Wednesday that he believed contamination of the sea had been continuing since the March 2011 catastrophe.

"I think contamination of the sea is continuing to a greater or lesser extent," Tanaka said.

"It was contaminated at the time of the accident, but I think it has been continuing for the last two years. Coming up with countermeasures against all possible scenarios is a top priority."

Fukushima's operator, Tepco, has acknowledged problems are mounting at the plant north of Tokyo, the site of the world's worst atomic disaster since Chernobyl in 1986.

Tepco said it was checking Tanaka's comments and could offer no immediate comment.

On Tuesday, the company said radiation levels in groundwater had soared, suggesting highly toxic materials from the power station were getting closer to the Pacific more than two years after three meltdowns triggered by a catastrophic earthquake and tsunami.

Observation between the damaged Reactor No 2 and sea showed levels of radioactive caesium-134 and ceasium-137 had soared over the weekend, the company said.

In the days after the March 2011 earthquake and tsunami, a plume of radiation from explosions fell over wide areas of the land and sea.

Toxic materials, such as caesium, were later found to have leaked through channels in the ground on the side of the station by the sea.