A new explosion rocked Japan's stricken Fukushima Daiichi nuclear power complex on Monday, sending a plume of smoke into the air.
Japan's nuclear safety agency said it could not confirm whether or not the hydrogen explosion at the plant's No.3 reactor had led to an uncontrolled leak of radioactivity.
However, the blast did not damaged the plant's No.3 reactor vessel, news agency Jiji said, quoting the plant's owner,
Tokyo Electric Power Co.
Authorities have declared an evacuation zone within a 20 km (12 mile) radius of the plant and evacuated 210,000 people.
"We have strongly advised all the people still within the evacuation area to go inside nearby facilities," said nuclear safety agency spokesman Ryo Miyake.
Some 746 people -- patients, elderly people and care workers at three hospitals and nursing homes -- remained within the 20 km area on Monday.
Operators had earlier halted injection of sea water into the reactor, resulting in a rise in radiation levels and pressure.
The government had warned that an explosion was possible because of the buildup of hydrogen in the building housing the reactor.
Japan battled through the weekend to prevent a nuclear catastrophe and to care for the millions without power or water in its worst crisis since World War Two, after a huge earthquake and tsunami that likely killed more than 10,000 people.
A badly wounded nation has seen whole villages and towns wiped off the map by a wall of water, leaving in its wake an international humanitarian effort of epic proportions.
Kyodo quoted authorities as saying the new tsunami could be up to a height of three metres and issued an alert for the country's Pacific coast, including Fukushima prefecture.
As the country returned to work on Monday, markets began estimating the huge economic cost, with Japanese stocks plunging around 5 percent and the yen falling against the dollar.
Prime Minister Naoto Kan said the situation at the crippled Fukushima nuclear plant remained worrisome and that the authorities were doing their utmost to stop damage from spreading.
(With AFP inputs)