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Japanese military helicopters dumped seawater on a stricken nuclear reactor on Thursday morning, to cool overheated fuel rods inside its core.

Japanese news agency NHK broadcast pictures of a helicopter dropping a load of water on Unit 3, one of six reactors at the Fukushima Dai-ichi nuclear power plant.

A defence ministry spokeswoman said a Japanese military CH-47 Chinook helicopter began dumping seawater on the damaged reactor at 9:48 a.m.

The helicopters took off from Kasumime Air Base in Sendai.

The spokeswoman said the dumping was intended both to help cool the reactor and to replenish water in a pool holding spent fuel rods.

However most of the water appeared to be dispersed in the air.

Reports reaching here from Tokyo suggest that reactors in the No.5 and No.6 units also saw an increase in temperatures, although there have been no reports of abnormalcies or explosions.

Earlier on Wednesday, Japan's nuclear safety agency said 70 percent of the nuclear fuel rods may have been damaged at Fukushima Daiichi No.1 reactor that was first stricken last week, triggering the crisis.

Chief Cabinet Secretary Yukio Edano was quoted by media reports as saying that at the moment, detectors showed 400 millisieverts per hour near the No.3 reactor, and 100 millisieverts at No.4 reactor,

Exposure to over 100 millisieverts a year is a level which can lead to cancer.

Meanwhile, US officials in Washington, warned that the Fukushima Dai-ichi plant in northeastern Japan may be on the verge of spewing more radioactive material because water was gone from a storage pool that keeps spent nuclear fuel rods from overheating.

The troubles at several of the plant's reactors were set off when last week's earthquake and tsunami knocked out power and ruined backup generators needed for their cooling systems.

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Tags: Japan