The operator of Japan's crippled Fukushima nuclear plant has admitted for the first time that radioactive groundwater has leaked out into the Pacific Ocean.

The Tokyo Electric Power Company (TEPCO) has been battling for months to stop groundwater leaking into the complex and becoming contaminated.

Now the company has admitted that radioactive water from the site has leaked into the sea, although a spokesman insists the impact on the ocean will be limited.

Earlier this month TEPCO revealed that groundwater samples taken at the Fukushima plant showed that levels of radioactive caesium had increased by more than 100 times in just a few days.

The company acknowledged that it had no explanation for the soaring figures.

The head of Japan's Nuclear Regulation Authority believes contamination of the sea has been continuing since the explosions at Fukushima following the 2011 tsunami.

TEPCO had previously failed to confirm the ground water leakage, more than two years after the worst nuclear accident since Chernobyl in 1986.

"We would like to offer our deep apology for causing grave worries for many people, especially for people in Fukushima," Masayuki Ono, TEPCO's general manager, told a news conference.

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.