TOKYO (AFP) - Steam has been spotted near a pool storing machinery removed from a crippled reactor at the Fukushima nuclear power plant, its Japanese operator said on Thursday.

"Steam has been seen around the fifth floor of the Reactor 3 building," a Tepco spokesman told AFP.

The roof of the building was blown off in a hydrogen explosion in the days after the March 2011 meltdowns.

"(The steam) was drifting thinly in the air and it's not like a big column of steam is spurting up," the spokesman said.

"Neither the temperature of the reactor nor readings at radiation monitoring posts have gone up.

"We do not believe an emergency situation is breaking out although we are still investigating what caused this." The pool is on the fifth floor and stores devices and equipment removed from the reactor.

The incident is the latest in a growing catalogue of mishaps at Fukushima that have cast doubt on Tepco'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 rat that left cooling pools without power for more than a day.