The operator of Japan's crippled Fukushima nuclear plant said Friday it still did not know what caused steam seen inside a reactor building, nor why it was no longer there.

Tokyo Electric Power (TEPCO) spotted water vapour around the fifth floor of the building housing the badly-damaged Reactor 3 on Thursday.

The company said it was looking at the possibility that accumulated rainwater had been the cause, but admitted Friday it still did not know for sure.

TEPCO, which has faced repeated criticism for playing down problems or not being open about the difficulties it faces at Fukushima, said the steam did not contain an abnormally high level of radiation.

"We still don't know what caused the steam and are currently investigating," said a spokesman.

"Workers will today use a remotely-controlled crane and measure the temperature of the area."

The reactor, broken by a massive tsunami in March 2011, is too dangerous to approach but a camera feed showed steam was no longer visible on Friday morning, the utility said.

The roof of the building was blown off in a hydrogen explosion days after meltdowns that were sparked when cooling systems were flooded by the deadly waves.

TEPCO on Thursday said monitoring equipment at the plant indicated no change to the amount of toxic chemicals it is releasing.

Steam was also sighted at least once in July last year, a company spokesman told reporters Thursday, but added the information was not made public and the vapour disappeared in a short time.

The latest case served as a further reminder about the continuing precariousness of the facility.

A series of leaks of water contaminated with radiation have shaken confidence, as did a blackout caused by a rat that left cooling pools without power for more than a day.

The company has admitted in recent weeks that groundwater and soil samples taken at the plant are showing high readings for potentially cancer-causing isotopes, including caesium-137, tritium and strontium-90.

Japan's nuclear watchdog said last week the plant was very likely leaking highly radioactive substances into the Pacific Ocean.

Tens of thousands of people were forced from their homes by the threat of radiation in the immediate aftermath of the disaster, with many still unable to return.

While the natural disaster claimed more than 18,000 lives, no one is officially recorded as having died as a direct result of the radiation released by the disaster.