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Steam has been seen rising from a reactor building at Japan's Fukushima nuclear plant, its operator says.

Tokyo Electric Power Company (Tepco) said there was no emergency situation and there were no signs of increased radiation in the area.

It says it is investigating what is causing the steam at the damaged No 3 reactor building.

The plant, crippled by the earthquake and tsunami in 2011, has seen a series of water leaks and power failures.

The tsunami knocked out cooling systems to the reactors, three of which melted down.

Water is being pumped into the reactors to cool them, but that has left Tepco with the problem of storing the contaminated waste water.

'Monitor closely'

A worker first noticed the steam after reviewing camera footage taken of the building, Tepco said.

The operator there was a "steam-like gas wafting through the air near the central part of the fifth floor [equipment storage pool side]" of the No 3 reactor building.

The reactor water injection and the cooling of the spent fuel pool were "continuing stably", Tepco said. There were also no significant change in the temperature of the reactor.

"We will continue to monitor the status closely," the statement added.

"We do not believe an emergency situation is breaking out although we are still investigating what caused this," a spokesman told Agence-France Presse news agency.

Mayumi Yoshida, another Tepco spokesperson, told Reuters news agency: "We think it's possible that rain made its way through the reactor building and having fallen on the primary containment vessel, which is hot, evaporated creating steam."

This is the latest in a series of problems that the Fukushima power plant has faced in recent months.

Last week, a sharp increase in radioactive cesium was detected in groundwater 25m (82ft) from the sea.

In June, radioactive water was also found to be leaking from a storage tank.

Experts say years of work lie ahead before the problems at the plant can be fully contained.