The operator of the Fukushima nuclear plant in Japan said that steam had been spotted at the battered reactor for the second time in days and disappeared in hours.

Steam was seen around the fifth floor of the building housing Reactor No 3 Tokyo Electric Power Co (TEPCO) said on Tuesday.

The steam has raised concerns about the damaged reactor, but TEPCO said no significant changes occurred, including in the levels of potentially cancer-causing radioactivity the broken reactor is releasing.

Steam was spotted in the same area on Thursday last week but had disappeared by the next day without any cause known. TEPCO said it was looking at the possibility that accumulated rainwater had been the source.

The workers continue with the operation to inject cooling water into the reactor and a pool storing nuclear fuel.

TEPCO admitted last Monday that Fukushima was likely leaking contaminated water into sea. The confirmation of the leak, is the first official acknowledgement of what experts have been suspecting for some time.

The operator of the stricken plant was repeatedly denying any contaminated water had reached the sea, despite spikes in radiation levels in samples.

The company is currently injecting chemical solution into the coastline embankment to solidify underground structure and block contaminated underground water from escaping into sea.

A 9.0 earthquake and the resulting tsunami in March 2011 killed nearly 20,000 people and set off the world's worst nuclear crisis in 25 years when the Fukushima plant was destroyed causing reactor meltdowns, hydrogen explosions and leaking radiation into the sea and air.