The operator of Japan's Fukushima nuclear plant insisted on Thursday there was no sign of spiking radiation at the crippled facility, after steam was found in a reactor building.

Tokyo Electric Power Co (TEPCO) said it believed rainwater sitting on the reactor was the source of the vapour, which did not contain an abnormally high level of radiation. TEPCO also insisted that monitoring equipment indicated no change to the status of the plant or to the amount of toxic materials it is releasing.

But the case served as a reminder about the continuing precariousness at the facility more than two years after it was wrecked by a tsunami that sent the plant into meltdown.

Direct access to the reactor is incredibly difficult because radiation levels make it unsafe for people to go near.

However, a worker watching a camera feed of the site spotted the steam in the building housing Reactor 3. 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.

Steam was also sighted at least once in July last year, TEPCO spokesman Masayuki Ono told reporters, but added the information was not made public and the vapour disappeared in a short time. TEPCO has faced repeated criticism for playing down problems or not being open about the difficulties it is facing at Fukushima.Relatively cool, rainy weather might have prompted the steam seen Thursday, Ono said.

"We believe it is highly likely that rainwater caused the steam," he said.

Neither the temperature of the reactor nor readings at radiation monitoring posts had risen in the hours after the steam was first seen, and the reactor remained subcritical. Criticality is the term used for reactors in which there is a self-sustaining nuclear chain reaction.AFP