Japan's nuclear watchdog will beef up monitoring at the crippled Fukushima power plant, an agency official said Tuesday, after leaks of radioactive groundwater into the ocean sparked heavy criticism of the site's operator.

The leaks have triggered alarm bells over the plant's precarious state and the capability of Tokyo Electric Power (TEPCO) to deal with a long list of problems two years after it was swamped by a tsunami that sent reactors into meltdown.

The Nuclear Regulation Authority (NRA) plans to pull together two dedicated teams to investigate water contamination and its impact on the ocean's ecosystem, according to the the official.

"We still don't know the root cause of the problems as they are more complicated than initially thought," the NRA official said.

A formal announcement on the new teams, expected to be comprised of experts from the regulator, TEPCO and a government-back research institute, is expected on Wednesday.

While the NRA has largely concentrated on the safety of restarting Japan's nuclear reactors -- just two of 50 are currently in operation -- TEPCO has been in charge of the Fukushima clean-up.

Last week, the utility stated for the first time that radioactive groundwater had leaked outside the plant, confirming long-held suspicions of ocean contamination from the shattered reactors.

The company had previously reported rising levels of cancer-causing materials in groundwater samples taken from underneath the plant itself, but maintained it had halted toxic water from leaking beyond its borders.

As Japan's nuclear watchdog heaped doubt on TEPCO's claims, it eventually admitted delaying the release of test results confirming the leaks.

On Friday, a pair of foreign nuclear experts who are members of a Fukushima monitoring panel, blasted TEPCO over its lack of transparency.

It "appears that you are not keeping the people of Japan informed", Dale Klein, former head of the US Nuclear Regulatory Commission, told the panel during a visit to Tokyo.

"These actions indicate that you don't know what you are doing... you do not have a plan and that you are not doing all you can to protect the environment and the people."

The blunt criticism comes after a litany of ongoing problems at the site of the worst atomic accident in a generation.

Tens of thousands of residents were evacuated and many may never be able to return home with decommissioning of the plant expected to take decades.