The operator of the crippled Fukushima nuclear plant in Japan said Tuesday steam had been spotted at the battered reactor for the second time in days, but levels of radioactivity had not risen.

Steam was seen around the fifth floor of the building housing Reactor No. 3 shortly after 9:00 am (0000 GMT), Tokyo Electric Power Co (TEPCO) said, adding workers were continuing with the ongoing operation to inject cooling water into the reactor and a pool storing nuclear fuel.

TEPCO said monitoring equipment showed no significant changes, 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, with TEPCO saying it did not know for sure what had caused it.

It said it was looking at the possibility that accumulated rainwater had been the source.

TEPCO said the steam had disappeared by early afternoon.

"We judged that the steam is not present anymore, after not having observed it from 1:30 pm to 2:30 pm," the utility said in a statement, adding they would continue to examine possible causes.

The reactor, devastated by a massive tsunami in March 2011, is too dangerous to approach, and workers had seen the steam on a camera feed, 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 as the tsunami swept ashore.