A major aftershock rocked northeast Japan on Thursday and a tsunami warning was issued for the coast devastated by last month's massive quake and tsunami that crippled a nuclear power plant.
The warning was later lifted and no tsunami were reported. No damage from the quake, measured at magnitude 7.4 by the Japan Meteorological Agency, was detected at the Fukushima Daiichi plant, operator Tokyo Electric Power Co (Tepco) said.
Workers struggling to bring the plant under control were evacuated soon after the aftershock struck, shortly before midnight.
Large parts of northern Japan, where infrastructure was severely damaged by the March 11 quake and tsunami, were without electricity following the latest of many aftershocks, the biggest since last month's killer quake.
Toru Hanai, a Reuters photographer in Oshu, Iwate prefecture, near the epicentre of Thursday's aftershock, said his hotel lost power and a water pipe burst.
"Everything fell. My room is a complete mess and power is widely out in this area," he said.
In the capital, Tokyo, buildings also shook.
"It started out as nothing much, then the building started swaying quite strongly," a Reuters witness said.
As of 01:30am (1630 GMT) seven people were reported injured, two of them seriously, a spokesman for the National Police Agency said.
Last month's huge 9.0 magnitude quake triggered tsunami waves which swept in along the coast, wiping out towns. About 28,000 people were killed or are missing.
The disaster also disrupted industry and affected supply chains around the world but it was not immediately clear if Thursday's aftershock would compound those problems.
At the Fukushima nuclear plant, Tepco said it was continuing to inject nitrogen into reactor No 1 after no irregularities were reported.
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.
There were no abnormalities in radiation levels around Tohoku Electric's Onagawa nuclear power plant, where fuel rods are being cooled with just one outside power source, Japan's nuclear safety agency said.
As well as Fukushima Daiichi and Onagawa, nuclear power plants Higashidori in Aomori prefecture, Tokai No 2 in Ibaraki prefecture, and Fukushima Daini have been out of operation since the March quake.
No abnormalities were reported at those plants after Thursday's quake, which the meteorological agency said was an aftershock from last month's quake.
"Due to the (March 11 quake), the risk of landslides or buildings collapsing is higher than usual and there are possibilities of further damage with aftershocks," deputy chief cabinet secretary Tetsuro Fukuyama told reporters on Friday.
Japan's neighbours have sounded increasingly alarmed over the risk of radiation from the damaged plant 240 km (150 miles) north of Tokyo, 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.
Tepco said late on Thursday it did not expect it would have to dump any more contaminated water into the ocean after Saturday.
Earlier, Tepco said the chance of a repeat of the gas explosions that damaged two reactors in the first days of the disaster was "extremely small".
But as engineers battle multiple crises -- some the result of efforts to try to cool reactors -- officials admit it could take months to bring the reactors under control and years to clear up the toxic mess left behind.
"Data shows the reactors are in a stable condition, but we are not out of the woods yet," chief cabinet secretary Yukio Edano told reporters.
The government has already set up a 20 km (12 miles) 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.
Earlier, China's health ministry said traces of radioactivity in spinach had been found in three provinces.
In South Korea, some schools closed because parents were worried that rain could be toxic.
"We've sent out an official communication today that schools should try to refrain from outdoor activities," an education official in South Korea said.
South Korea's nuclear safety agency reported a small level of radioactive iodine and caesium particles in rain but said it was not enough to be a health concern. The few schools that closed were expected to reopen on Friday if the rain stopped.
India said a blanket ban on food items imported from Japan was not warranted, though authorities would monitor the situation every week, a source in the trade ministry said.
India said on April 5 it had imposed a three-month ban on imports of food from Japan on fears that radiation from an earthquake-hit nuclear plant was spreading to other parts of the country.
($1=85.475 Japanese yen)