Fukushima's highly radioactive groundwater could already be spreading out to the Pacific Ocean, Japan's nuclear watchdog, the Nuclear Regulation Authority said on Wednesday.

Fukushima Highly Radioactive Groundwater Believed to Already Spreading to Ocean

"It is strongly suspected that highly concentrated contaminated waste water has leaked to the ground and has spread to the sea," the authority said in its written review following the announcements issued by Tokyo Electric Power (TEPCO) that it can't find the source of the recent increase in radiation levels in test wells at the Fukushima Daiichi nuclear plant.

At the same time, the agency hinted the government and experts may need to double check Tepco's data and if it should be taken as it is.

"We need to consider whether we should be relying only on data provided by Tepco, and check for ourselves the level of contamination in seawater," agency Chairman Shunichi Tanaka said at a meeting releasing the report.

Samples of groundwater taken on Tuesday at the Fukushima plant showed levels of possibly cancer-causing caesium-134 were more than 110 times higher compared from those taken on Friday.

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

Manabu Fukumoto, professor of Tohoku University's Medical School, told the Wall Street Journal that underground water moves through layers and eventually goes to the sea. "The process sometimes takes a thousand years. During the time, it will be necessary to keep monitoring."