An operation to clean-up radioactive water at Japan's Fukushima nuclear power plant has been suspended, hours after it got under way.

The plant's operator blamed radiation levels, which began rising dramatically, for the suspension.

Tokyo Electric Power Company had undertaken the operation at the plant, disabled by the 11 March earthquake and tsunami, to avert spillage into the sea of large, growing pools of radioactive water.

A statement issued by TEPCO said the suspension, five hours after the operation began, was prompted by a sharp rise in radiation in a part of the system intended to absorb caesium.

‘At the moment, we haven't specified the reason,’ a TEPCO spokesman told a news conference. ‘So we can't say when we can resume the operation. But I'd say it's not something that would take weeks.’

The official said teams working at the plant believed the radiation rise could be linked to sludge flowing into the machinery intended to absorb cesium. Another cause could be pipes surrounding it.

But resumption, he said, was critical to deal with the highly radioactive water. Officials say 110,000 tonnes, the equivalent of 40 Olympic swimming pools, is stored there.

‘Unless we can resume the operation within a week, we will have problems in disposing of the contaminated water,’ the official said.

‘But if this is caused by the reasons we are thinking, we can resume the operation within a week.’

The official said TEPCO foresaw no delay in its overall plan to bring the Fukushima Daiichi plant fully under control by the end of the year.

The plan calls for a shutdown of its three unstable reactors by January 2012.

The clean-up operation had got under way yesterday after being delayed by a series of glitches at the plant, which is 240km northeast of Tokyo.

Officials had said earlier this week that large and growing pools of radioactive water at the plant were in danger of spilling into the sea within a week unless action was taken quickly.

The company said the decontamination process could produce large amounts of radiation sludge, causing a further problem for its long-term storage.

Meanwhile, a 5.9 magnitude earthquake struck 110km east of Fukushima near the east coast of Honshu at a depth of 30km, the Japan Meteorological Agency said.

No tsunami warnings were in effect and there were no immediate reports of damage.