Japan said it was operating on an atomic power emergency footing but said no radiation leaks were detected among its reactors after an 8.9 magnitude earthquake struck Friday, triggering a huge tsunami.
Prime Minister Naoto Kan declared the emergency to enable authorities to implement emergency measures. Residents living near plants were not required to take special action, Chief Cabinet Secretary Yukio Edano told a news conference.
"We have declared a nuclear emergency state to take every possible precaution," Edano said. "Let me repeat that there is no radiation leak, nor will there be a leak."
"We ask residents in the areas near power plants to act calmly."
A fire broke out in the turbine building of Onagawa nuclear plant in Miyagi Prefecture, but operator Tohoku Electric Power said there were no indications of a radioactive leak, Kyodo News reported.
Miyagi prefecture was one of the areas worst hit by the tsunami.
Kyodo also reported that an emergency core-cooling unit had been activated at Fukushima nuclear plant, without giving further details.
Earlier Friday Prime Minister Naoto Kan had said no radiation leaks have been detected from Japan's nuclear power stations after the massive quake struck the country.
Four Japanese nuclear power plants closest to the epicentre of the quake have been safely shut down, the UN atomic watchdog said Friday.
The quake struck just under 400 kilometres (250 miles) northeast of Tokyo, the US Geological Survey said. It was followed by more than a dozen aftershocks, one as strong as 7.1.