Medical officials check for signs of radiation on children from the evacuation area near the Fukushima Daiichi nuclear plant in Koriyama, Japan, on Sunday. Kim Kyung-Hoon / Reuters

A Chinese rescue team, comprised of 15 members, prepares to leave for Japan at Beijing Capital International Airport on Sunday morning. It is the first time the Japanese government has accepted assistance from China for disaster relief. Ren Zhenglai / Xinhua

Japanese official says it might have occurred

160 people possibly exposed to radiation

10,000 feared dead after tsunami hit towns

Millions of people without power and water

FUKUSHIMA, Japan - Japan fought on Sunday to avert a meltdown at two earthquake-crippled nuclear reactors, describing the massive quake and tsunami, which may have killed more than 10,000 people, as the nation's biggest crisis since World War II.

The country is struggling to respond to a disaster of epic proportions, with millions of people without water or power and whole towns wiped off the map.

The Japanese Meteorological Agency upgraded the magnitude of Friday's massive earthquake from 8.8 to 9.0 on Sunday, the strongest ever recorded in Japan.

"The earthquake, tsunami and the nuclear incident have been the biggest crisis Japan has encountered in the 65 years since the end of World War II," a grim-faced Prime Minister Naoto Kan told a news conference.

"We're under scrutiny on whether we, the Japanese people, can overcome this crisis."

He said the nation's future will be decided by the choices made by each Japanese person and urged all to join in their determination to rebuild the nation.

As he spoke, officials worked desperately to stop fuel rods in the damaged reactors from overheating, which could in turn melt the container that houses the core, or even explode, releasing radioactive material into the wind.

A complete meltdown - the collapse of a power plant's ability to keep temperatures under control - could release uranium and dangerous contaminants into the environment and pose major, widespread health risks.

Chief Cabinet Secretary Yukio Edano said on Sunday that a hydrogen explosion could occur at No 3 reactor of the Fukushima Daiichi nuclear complex, the latest reactor to face a possible meltdown.

That follows a blast the day before in the power plant's No 1 reactor, and operators attempted to prevent a meltdown there by injecting seawater into it.

"At the risk of raising further public concern, we cannot rule out the possibility of an explosion," Edano said.

He said there might have been a partial meltdown of the fuel rods at the No 1 reactor. Engineers were pumping in seawater, trying to prevent the same happening at the No 3 reactor.

"The use of seawater means they have run out of options," said David Lochbaum, director of the Union of Concerned Scientists Nuclear Safety Project.

The operator of the nuclear reactors said on Sunday the top of fuel rods at the reactors had been three meters above water, an indication of a possible meltdown.

Tokyo Electric Power warned that the No 3 reactor was overheating and that so much of the cooling water had briefly evaporated that mixed oxide fuel rods were exposed to the air, Kyodo news agency said.

"Radiation has been released in the air, but there are no reports that a large amount was released," Jiji news agency reported.

Up to 160 people might have been exposed to radiation, said Ryo Miyake, a spokesman from Japan's nuclear agency. The severity of their exposure, or if it had reached dangerous levels, was not clear.

Broadcaster NHK, quoting a police official, said more than 10,000 people may have been killed.

Almost 2 million households were without power in the freezing north, the government said. There were about 1.4 million without running water.

Kyodo said about 300,000 people were evacuated nationwide, many seeking refuge in shelters, wrapped in blankets, some clutching each other, sobbing.

Tokyo Electric Power says it will ration electricity with rolling blackouts in parts of Tokyo and other Japanese cities.

Deng Wei, Chinese embassy spokesman in Tokyo, said on Sunday that there are, so far, no reports of Chinese casualties but Kyodo reported that 40 Chinese trainees sent by a service company in Shandong province have lost contact after the earthquake. Embassy staff members are working hard to track all Chinese citizens in Japan, Deng said.

Authorities in Japan have set up a 20-km exclusion zone around the Fukushima Daiichi plant and a 10-km zone around another plant nearby. About 170,000 people have been moved out, while authorities prepared to distribute iodine to protect people from radioactive exposure.

The nuclear accident, the worst since the Chernobyl disaster in 1986, sparked stinging criticism that authorities were ill-prepared for such a massive quake and the threat that could pose to the country's nuclear power industry.

Kan said on Sunday the crisis was not the same as Chernobyl.

"Radiation has been released in the air, but there are no reports that a large amount was released," Jiji news agency quoted him as saying. "We are working to prevent damage from spreading."

Japanese officials said they had also ordered up the largest mobilization of their Self-Defense Forces since World War II to assist in the relief effort.

The Chinese International Search and Rescue Team arrived in Japan on Sunday - the first time the government has accepted assistance from China for disaster relief.

The 15 team members took with them four tons of materials and equipment for search and rescue, power supply and telecommunication. They began their work in Iwate prefecture. More than 30,000 Chinese nationals are estimated to live in Iwate and the other two most seriously damaged prefectures, Miyagi and Fukujima.