A strong earthquake of magnitude 7.1 shook the northeast of Japan tonight and a tsunami warning was issued for the coast already devastated by last month's massive quake and the tsunami that crippled a nuclear power plant.
The tsunami warnings for the northeast coast were later lifted.
Japan is struggling to bring the Fukushima Daiichi plant under control after the March 11th quake and tsnumai, which killed, or left missing, about 28,000 people.
No damage was detected at the Fukushima plant from today’s earthquake and NHK said workers had been evacuated without reports of any injuries.
There were no immediate reports of injuries or damage elsewhere but people in areas covered by the tsunami warning should evacuate to higher ground, Japan's NHK public television said.
Japan's neighbours have sounded increasingly alarmed over the risk of radiation from the plant, while tourists are staying away in what should be the peak season, and the country seeks ways to cut power use.
The world's worst nuclear disaster in 25 years is also raising concern over safety in the United States, which has more atomic reactors than any other country, especially at one plant which is similar to the one in Fukushima wrecked by last month's 9.0 magnitude earthquake and tsunami.
Engineers, who sealed a leak this week that had allowed highly radioactive water into the sea, are pumping nitrogen into one reactor to prevent the risk of a hydrogen gas explosion, and want to start the process in another two reactors.
Operator Tokyo Electric Power Co (Tepco) said it did not expect it would have to dump any more contaminated water into the ocean after Saturday. Earlier, it said the chance of a repeat of the gas explosions that damaged two reactors in the first days of the disaster was "extremely small".
The government has set up a 20km exclusion zone around the plant, banned fishing along much of the northeast coast and set up evacuation centres for the tens of thousands forced to leave their homes following the crisis.
Trace levels of radioactive material have been detected in the air in 22 Chinese provinces but the amounts did not pose a threat to health or the environment, China's state news agency Xinhua said.
In South Korea, some schools closed because parents were worried that rain could be toxic.