Steam has been spotted in a reactor building at the crippled Fukushima nuclear plant in Japan, its operator said Thursday, 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, 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 quake and tsunami.
TEPCO spokesman Koichiro Shiraki said that neither the temperature of the reactor nor readings at radiation monitoring posts had risen six hours after the steam was first spotted, and that the reactor remained subcritical. Criticality is the term used for reactors in which there is a self-sustaining nuclear chain reaction.
The steam was seen near a pool on the fifth floor that stores devices and equipment removed from the reactor before the disaster as part of regular operations. The pool sits next to the 3 centimetre-thick (1.2 inch) steel structure enclosing the reactor.
Shiraki said one possible cause was rain, which could have evaporated as it came in contact with the reactor container vessel.
"This is just a possibility... but rainwater could get into or near the storage pool," he said, adding it had rained there at night and early Thursday morning.
The temperature of the container was about 38 degrees Celsius (100 Fahrenheit) early Thursday, according to the company.
Workers were continuing to pump water into the reactor and fuel pool as part of continuing cooling efforts, TEPCO said, adding it would measure dust near the building as well as the air above it to gauge radiation levels.
A 9.0 earthquake and the resulting massive tsunami in March 2011 knocked out cooling systems at Fukushima Daiichi, causing the meltdowns of reactors.
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.
The company has admitted in recent weeks that water and soil samples taken at the plant are showing high readings for potentially dangerous isotopes, including caesium-137, tritium and strontium-90.
Japan's nuclear watchdog said last week the Fukushima reactors are very likely leaking highly radioactive substances into the Pacific Ocean.
Members of the Nuclear Regulation Authority voiced frustration at TEPCO, which has failed to identify the source and the cause of rising readings in groundwater.
The authority's officials are urging TEPCO to offer more detailed and credible data and to try to better explain to the public what it knows.
Most of Japan's nuclear reactors remain off-line, largely due to public distrust of the industry.
Prime Minister Shinzo Abe's Liberal Democratic Party, as well as utilities, are hoping to restart them.