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Japan scrambled on Saturday to contain a crisis at two nuclear plants damaged when a massive earthquake and tsunami struck its northeast coast. There have been conflicting reports about a partial meltdown at the Fukushima nuclear power plant, north of Tokyo.

Reports suggest that technicians are battling to cool Unit 3 reactor with sea water, following a blast at the building housing Unit 1 reactor on Saturday.

Media reports further suggest that more than 170,000 people evacuated the quake- and tsunami-savaged northeastern coast where fears spread over possible radioactive contamination.

"Now number three reactor valve might be showing some failure.... and the failure in the valve has to be solved.... and the air pressure inside has to be lowered.... and we are working on that," a Japanese technician was quoted as saying.

The reactor faced serious troubles after its emergency cooling system failed, triggering a fresh radiation threat.

The magnitude 8.9 quake hit at 2.46pm, centred offshore of the city of Sendai on the eastern cost of Honshu island. Serious secondary effects followed including a significant tsunami and a major fire at a fossil fuel installation.

However, Japan's government spokesman says the metal container sheltering a nuclear reactor was not affected by an explosion that destroyed the building it's in.

Yukio Edano says the radiation around the plant did not rise after the blast but instead is decreasing. He added that pressure in the reactor was also decreasing.

Pressure and heat have been building at the nuclear reactor since an earthquake and tsunami Friday caused its cooling system to fail.

The government has ordered people within a 20-kilometre radius of the plant in Fukushima to evacuate the area.

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