Image caption The Fukushima plant has faced a series of problems since the 2011 crisis

Six workers at Japan's Fukushima nuclear plant have been doused in radioactive water, its operator says.

The incident happened after a worker removed a pipe connected to a water treatment system at the plant, Tokyo Electric Power Company (Tepco) said.

A senior official from Japan's nuclear watchdog said he did not believe the workers had received a "seriously troubling" dosage of radiation.

Fukushima has been hit by a series of toxic water leaks in recent months.

The plant was badly damaged by the earthquake and tsunami of March 2011. Cooling systems for reactors were knocked out, causing meltdowns at three of them.

Water is being pumped in to cool the reactors. However, this creates large amounts of contaminated water that must be stored securely.

Previous Fukushima problems 7 Oct A plant worker accidentally switches off power to pumps used for cooling damaged reactors

A plant worker accidentally switches off power to pumps used for cooling damaged reactors 3 Oct Tepco says there is a radioactive water leak after workers overfill a storage tank

Tepco says there is a radioactive water leak after workers overfill a storage tank 21 Aug Japan's nuclear agency upgrades Fukushima alert level

Japan's nuclear agency upgrades Fukushima alert level 20 Aug Tepco says 300 tonnes of radioactive water has leaked from a storage tank into the ground

Tepco says 300 tonnes of radioactive water has leaked from a storage tank into the ground July Tepco for the first time admits radioactive water is going into the sea

Tepco for the first time admits radioactive water is going into the sea June Tepco says radioactive water leaking from a storage tank to the ground

Tepco says radioactive water leaking from a storage tank to the ground April Tepco suspects a fresh radioactive water leak at Fukushima

Tepco suspects a fresh radioactive water leak at Fukushima March Tepco suspects a rodent may have been behind a power cut that shut down cooling systems

Tepco suspects a rodent may have been behind a power cut that shut down cooling systems Dec 2011 Contaminated water leaks from a treatment system, caused by a crack in the foundation

Some of the water has leaked from the tanks, pipes and damaged structures, leading to concerns contaminated water is mixing with groundwater that is flowing into the sea.

'Carelessness'

The detached pipe was connected to a desalination system, Tepco said, which was being used to treat toxic water before storage in tanks.

Tepco said it was measuring the radiation that the workers, who were wearing protective gear, were exposed to.

"It is serious in that it was another problem caused by carelessness, but I do not believe it is a seriously troubling dosage," Shunichi Tanaka, chairman of Japan's Nuclear Regulation Authority, was quoted as saying by Reuters news agency.

Japanese officials say the water to which the workers were exposed was emitting beta decay radiation - and that is important, says Jonathan Amos, the BBC's science correspondent.

Beta particles are very weakly penetrating, and the workers' protective overalls would have substantially limited their exposure. The reports suggest also that no water splashed in the faces of the clean-up staff, so there would have been little chance of contamination being ingested.

But the incident is just the latest in a series of mishaps at the plant, and it will no doubt bolster the view of those who believe Tepco is bungling the recovery operation, our correspondent adds.

The pipe had since been reconnected and the toxic water was contained, Tepco said.

The incident is the latest involving worker error. On Monday, it was reported that a plant worker accidentally switched off power to pumps used for cooling the damaged reactors.

Last week, workers overfilled a storage tank, and about 430 litres (100 gallons) of radioactive water may have leaked from the tank, with some possibly flowing into the sea, Tepco said.