FUKUSHIMA: Japanese engineers raced to prevent a meltdown at a stricken nuclear plant today, as rescuers scrambled to help millions left without food, water or heating by a devastating earthquake and tsunami.

A second explosion rocked the Fukushima nuclear complex yesterday and rapidly failing water levels exposed fuel rods in another reactor, but the United Nations' nuclear watchdog said the crisis was unlikely to turn into another Chernobyl.

Rescue workers combed the tsunami-battered region north of Tokyo, where officials say at least 10,000 people were killed in the 8.9-magnitude earthquake and tsunami that followed it.

"It's a scene from hell, absolutely nightmarish," said Patrick Fuller of the International Red Cross Federation from the northeastern coastal town of Otsuchi.

Prime Minister Naoto Kan has dubbed the multiple disasters Japan's worst crisis since the Second World War and, with the financial costs estimated at up to $180 billion, analysts said it could tip the world's third biggest economy back into recession.

The big fear at the Fukushima complex, 240km north of Tokyo, is of a major radiation leak. The complex has seen explosions at two of its reactors on Saturday and yesterday, which sent a huge plume of smoke billowing above the plant.

The worst nuclear accident since the Chernobyl disaster in Ukraine in 1986 has drawn criticism that authorities were ill-prepared and revived debate in many countries about the safety of atomic power.

Switzerland put on hold some approvals for nuclear power plants and Germany said it was scrapping a plan to extend the life of its nuclear power stations. The White House said US President Barack Obama remained committed to nuclear energy.

International Atomic Energy Agency head Yukiya Amano said the reactor vessels of nuclear power plants affected by the disaster remained intact and, so far, the amount of radiation that had been released was limited.

"Japanese authorities are working as hard as they can, under extremely difficult circumstances, to stabilise the nuclear power plants and ensure safety," Amano said, adding at a news conference later that it was "unlikely that the accident would develop" like Chernobyl.

The plant operator, Tokyo Electric Power Company (Tepco), said fuel rods at the No 2 reactor were fully exposed. This could lead to the rods melting down.

The rods, normally surrounded by cooling water, were partially exposed earlier after the engine-powered pump pouring in this water ran out of fuel. Tepco said it was preparing to pump more cooling water on the rods.

There were earlier partial meltdowns of the fuel rods at both the No 1 and the No 3 reactors, where the explosions had occurred. A Tepco official said the situation in the No 2 reactor was even worse than in the other units.

A meltdown raises the risk of damage to the reactor vessel and a possible radioactive leak.

"If cooling water is not returned, the core should melt in a matter of hours," said Edwin Lyman, senior scientist for global security programmes at the Union of Concerned Scientists which lobbies for stronger security and safety measures at nuclear plants.

Crucially, officials said the thick walls around the radioactive cores of the damaged reactors appeared to be intact after the earlier hydrogen blast.

But the government warned those still in the 20-km evacuation zone to stay indoors.

Nonetheless, US warships and planes helping with relief efforts moved away from the coast temporarily because of low-level radiation. The US Seventh Fleet described the move as precautionary.

South Korea, Hong Kong, Singapore and the Philippines said they would test Japanese food imports for radiation.

France's ASN nuclear safety authority said the accident could be classified as a level 5 or 6 on the international scale of 1 to 7, putting it on a par with the 1979 US Three Mile Island meltdown, higher than the Japanese authorities' rating.