Japan's Meteorological Agency says it has upgraded the magnitude of Friday's catastrophic earthquake to 9.0, which killed at least 1,600 people, and the number of missing people exceeded 20,000.
Japan raced to avert a meltdown of two reactors at a quake-hit nuclear plant Monday as the death toll from the disaster on the ravaged northeast coast was forecast to exceed 10,000.
Earlier, an explosion at the ageing Fukushima No. 1 atomic plant blew apart the building housing one of its reactors Saturday, a day after the biggest quake ever recorded in Japan unleashed a monster tsunami.
The atomic emergency escalated as crews struggled to prevent overheating at a second reactor where the cooling system has also failed, and the government warned that it too could suffer a blast.
Prime Minister Naoto Kan said the situation at the stricken power plant remained grave, and that Japan was facing its worst crisis since the end of World War II -- which left the defeated country in ruins.
"The current situation of the earthquake, tsunami and the nuclear plants is in a way the most severe crisis in the 65 years since World War II," Kan said in a televised national address Sunday.
"Whether we Japanese can overcome this crisis depends on each of us," said the premier, who was dressed in an emergency services suit.
Rolling power outages were due to start later Monday as the quake and tsunami crippled nuclear power plants in the northeast. Millions of people were left without electricity after the disaster hit Friday. Japan's nuclear industry provides about a third of its power needs.
Top government spokesman Yukio Edano said it was highly likely that a partial meltdown had occurred at the plant's number one reactor, and a second was possible at the plant 250 kilometres (160 miles) northeast of Tokyo.
"There is the possibility of an explosion in the number three reactor," he said, while voicing confidence that it would withstand the blast as the first reactor had.
A meltdown occurs when a reactor core overheats and causes damage to the facility, potentially unleashing radiation into the environment.
Edano said that some radiation had escaped in the accident, but that the levels released into the air were so far not high enough to affect human health.
Plant operator Tokyo Electric Power said that despite continuing efforts, it had not managed to ensure that the tops of the fuel rods in the two troubled reactors remained submerged. Exposed rods increase the risk of a meltdown.
France's Institute of Radiation Protection and Nuclear Safety (IRSN) said "very large" amounts of radioactivity were "produced simultaneously with the explosion" at Fukushima.
"During the explosion the rate of release at the edge of the site would have attained one millisievert (mSv) per hour," compared with naturally present radioactivity of 0.0001 mSv per hour, it said Sunday.
The government has said the radiation released into the air so far had not reached levels high enough to affect human health.
A cooling pump at another plant 120 kilometres from Tokyo, the Tokai No. 2, had failed, but a back-up was working and cooling the reactor, a plant spokesman said early Monday.
The United Nations said a total of 590,000 people had been evacuated in the quake and tsunami disaster, including 210,000 living near the Fukushima nuclear plants.
The colossal 9.0 magnitude tremor sent waves of churning mud and debris racing over towns and farmland in Japan's northeast, destroying everything in its path and reducing swathes of countryside to a swampy wasteland.
Chinese rescue team arrives Japan for quake relief
A 15-strong Chinese international search and rescue team Monday morning set out to join the relief work in Oofunato, Iwate, a city severely damaged by a 9.0-magnitude earthquake Friday.
The Chinese international search and rescue team, which arrived in Oofunato at around 10:30 p.m. Sunday night, is the first overseas team to join and help out the relief work in the city.
Team leader Yin Guangfui said they have brought along life exploration device and some other equipment to help carrying out the rescue.
The team, with good experience in search and rescue, will strive to do the relief work, Yin added.
The team members, including rescue and medical personnel, gathered at a primary school in Oofunato on Sunday and set off at 7:30 a.m. to the disaster-hit area together with the Japanese rescue teams after a briefing. The team is expected to be here for seven to 10 days.
The catastrophic earthquake hit northeast of Japan on March 11, triggering severe tsunami and causing extensive damage and loss in different areas. According to local media, a total of 985 people have been confirmed dead, 707 others injured and over 1,700 others missing.
Here is a series of satellite images show the city of Natori, Japan, indicating how the earthquake and tsunami that struck the region.
