The operator of Japan's stricken Fukushima nuclear power plant has said contaminated ground water had likely been flowing into the sea, acknowledging such a leakage for the first time.

Tokyo Electric Power Co, or Tepco, also came under fire on Monday for the revelation that the number of plant workers with thyroid radiation exposure times exceeding the threshold levels for increased cancer risks was ten times the number it had released previously.

We would like to offer our deep apology for causing grave worries for many people. Masayuki Ono, Fukushima plant general manager

Tepco has been repeatedly blamed for overlooking early signs, and covering up or delaying the disclosure of problems and mishaps.

The head of Japan's new Nuclear Regulation Authority, created since the March 2011 earthquake and tsunami wrecked Fukushima, said this month he believed contamination of the sea had been continuing since the accident.

The admission came when company general manager, Masayuki Ono, told news conference that plant officials have come to believe that radioactive water that leaked from the wrecked reactors and is likely to have seeped into the underground water system and escaped into sea.

Nuclear officials and experts have suspected a leak from the Fukushima Dai-ichi since early in the crisis, but Tepco had previously failed to confirm the ground water leakage more than two years.

"We would like to offer our deep apology for causing grave worries for many people, especially for people in Fukushima," Ono said.

Soaring radiation

Tepco said that based on water sample tests, any impact of the leakage appeared to be contained by silt fences erected near the devastated reactors.

The utility is already injecting the chemical sodium silicate into part of the seawall separating the ocean from the plant site to prevent ground water from seeping through.

The March 2011 earthquake and tsunami knocked out cooling systems at the Fukushima plant, triggering fuel meltdowns and causing radiation leakage, food contamination and mass evacuations.

Tepco this month acknowledged that levels of radiation in groundwater had soared, suggesting highly toxic materials from the plant were getting closer to the Pacific.