Steam has been spotted in a reactor building at the crippled Fukushima nuclear plant in Japan, its operator says, stressing there is no sign yet of increased radiation.
The incident, which Tokyo Electric Power Co (TEPCO) said was not "an emergency situation", is the latest underlining the plant's continuing precariousness more than two years after it was wrecked by a tsunami.
Steam has been seen around the fifth floor of the Reactor 3 building, a TEPCO spokesman told AFP on Thursday, adding it was "drifting thinly" and was not a large column of vapour.
"We do not believe an emergency situation is breaking out, although we are still investigating what caused this," he said.
The roof of the building was blown off in a hydrogen explosion in the days after the March 2011 meltdowns, which were sparked when cooling systems were flooded with seawater after a huge undersea 9.0 quake and tsunami.
Tens of thousands of people were forced from their homes by the threat of radiation.
TEPCO is struggling to manage the clean-up, which scientists say could take up to four decades to complete.
The steam is the latest in a growing catalogue of mishaps that have cast doubt on the utility's ability to fix the world's worst atomic disaster in a generation.
A series of leaks of water contaminated with radiation have shaken confidence, as did a blackout caused by a gnawing rat that left cooling pools without power for more than a day.
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