Japan's atomic watchdog summoned the boss of Fukushima operator TEPCO on Friday for a public dressing-down over sloppy standards at the crippled nuclear plant, as yet another problem with radiation-polluted water emerged.

The Nuclear Regulation Authority (NRA) called in Tokyo Electric Power Co president Naomi Hirose and other executives over "the inappropriate management of contaminated water".

"The problems have been caused by a lack of basic checks," NRA secretary general Katsuhiko Ikeda told Hirose.

"I can't help but say that standards of on-site management are extremely low at Fukushima Daiichi," Ikeda said. He added the utility should strengthen its staffing levels, including by sending workers from other nuclear plants.

Hirose apologised to Ikeda and pledged to increase efforts to deal with the ongoing problems "by using all the company's resources, including people, equipment and money".

The meeting came as it was revealed that a key system to decontaminate radioactive water at the plant had stopped again.

TEPCO said it halted the Advanced Liquid Processing System (ALPS) -- the only one of three that was in operation -- early Friday following "an alert suggesting abnormality in the process".

The firm said the cause of the problem was not known, but no leaks of radioactive water from the system had been detected. It was restarted in the evening, the company said.

The stoppage came just four days after TEPCO got the system back up and running following a breakdown when a piece of plastic clogged the machine.

ALPS is the great hope for TEPCO, which is struggling to cope with a huge -- and growing -- volume of liquid at the plant, where overheating reactors had to be cooled with thousands of tonnes of water after a tsunami hit in March 2011.

They continue to be doused, more than two-and-a-half years after the disaster.

"Whack-a-mole"

Without a functioning ALPS, TEPCO is dependent on only one separate decontamination system to begin processing about 1,000 tanks full of water.

Independent experts have said that ultimately this water will have to be dumped in the ocean once it has been scoured of the worst of its radioactive contamination.

But neighbouring countries, global pressure groups and local fishermen are deeply opposed to the idea, unmoved by assurances that the radiation will be massively diluted as it mixes with the vast Pacific.

Friday's stoppage is the latest in a growing list of setbacks at the plant, where TEPCO's haphazard and uncoordinated efforts to fix the mess have been derided by one government minister as akin to someone playing "whack-a-mole".

On Thursday the company announced 430 litres of polluted water had spilt from a tank as workers tried to remove rainwater dumped at the plant during a recent typhoon.

Its admission that the "contaminated water may well have flowed into the sea" came after the firm announced in August that 300 tonnes of toxic water that had leaked from a different tank had made its way into the ocean.

The UN atomic agency said Friday it was sending experts to Japan on October 14 for a week-long review of progress on the clean up.

After some time in Fukushima and meetings with government officials, the International Atomic Energy Agency will publish a report on its findings.

While the tsunami cost more than 18,000 lives, the nuclear disaster it caused is not officially recorded as having directly killed anyone.

But tens of thousands of people who were evacuated are still unable to return to their homes, with scientists warning some areas near the plant will have to be abandoned forever because of radioactive contamination.

Last week TEPCO applied to the Japanese watchdog for permission to restart two reactors at a different plant.

All of Japan's 50 working reactors are offline amid public scepticism over nuclear power.