TOKYO (AFP) - The cooling system for an undamaged reactor at the Fukushima nuclear power plant temporarily stopped working on Thursday, the plant's operator said, as mysterious steam continued to rise from a broken unit.

While the outage only last two hours and there was no danger of a runaway nuclear reaction, the incident further underlined the precarious state of the plant more than two years after it was hit by a tsunami.

Four of the six reactors at the Fukushima Daiichi plant were battered by the waves of March 2011, with their cooling systems knocked offline, sending three of them into meltdown.

Reactors 5 and 6 were not affected, although both still had fuel inside at the time of the disaster.

On Thursday morning machinery keeping Reactor 6 cool stopped as workers tested a back-up diesel generator, said operator Tokyo Electric Power Co (Tepco).

Meanwhile, steam that began appearing last week near the badly-wrecked Reactor 3 was still in evidence, the company said, admitting it was still no closer to identifying the reason for the vapour.

"Workers could still see the steam rising from the area as of 1pm (12 noon Singapore time)," a Tepco spokesman told AFP.

The company's best guess so far is that accumulated rainwater was being heated in some way. But, said the spokesman: "We still don't know for sure what the cause is." Thursday's incidents come after a week of bad news for TEPCO.

The utility was admonished by government officials on Tuesday for a delay in admitting that radiation-polluted groundwater was flowing into the sea.

It had reported rising levels of possibly cancer-causing materials in groundwater samples from underneath the plant, but maintained that toxic water was likely contained.

On Monday it admitted that its own study, completed days earlier, revealed the groundwater was leaking into the ocean, prompting criticism over the delay.

And on Thursday representatives of fishermen nationwide met senior company officials to express their anger at radioactive leaks into the sea.