Japan has officially asked the UN atomic watchdog to send a team of experts to help stave off a nuclear emergency following Friday's massive earthquake and tsunami.

Two explosions have already rocked the Fukushima plant and now fuel rods at the plant's No. 2 reactor are exposed and at risk of meltdown.

But as Japan struggles to deal with the aftermath of the earthquake and tsunami which are feared to have killed more than 10,000 people, government officials are playing down the nuclear concerns, insisting the situation is under control.

Authorities say the fuel rods in the reactor became fully exposed after the system that pumps cooling water through the reactor failed late last night.

Radiation and pressure levels jumped and engineers are trying to stabilise the situation.

Air pressure inside the reactor rose suddenly when the air flow gauge was accidentally turned off, blocking the flow of water and leading to full exposure of the rods, operator TEPCO said.

"We are not optimistic but I think we can inject water once we can reopen the valve and lower air pressure," a TEPCO official told reporters.

Japan's top government spokesman Yukio Edano said a major explosion was unlikely and insisted the government was not facing a Chernobyl-style disaster. He said engineers were pumping seawater in to stabilise the reactor and radiation around the plant was at tolerable levels.

A hydrogen explosion blew apart the building housing the plant's No. 1 reactor on Saturday, while a second explosion yesterday rocked the building housing the No. 3 reactor, injuring 11 people.

Authorities have declared an exclusion zone within a 20-kilometre radius of the plant and evacuated 210,000 people.

At one shelter a young woman holding her baby told public broadcaster NHK: "I didn't want this baby to be exposed to radiation. I wanted to avoid that no matter what."

This morning the head of the UN atomic watchdog, the International Atomic Energy Agency (IAEA), said it was "very unlikely" the Fukushima crisis would turn into a Chernobyl-like situation.

"Let me say the possibility that the development of this accident into one like Chernobyl is very unlikely," Yukiya Amano told a news conference at IAEA headquarters in Vienna.

Aftershocks are still frightening Japan, with another big tremor hitting Tokyo this morning. Buildings shook for about 10 seconds, keeping the already nervous population on edge.

'Scene from hell'

Many survivors spent another night without water, electricity, fuel or enough food as authorities appeared overwhelmed by the monumental scale of the disaster.

Emergency helicopters fly up and down the coast of devastated Miyagi prefecture, keeping an eye out for any further tsunamis and dropping rescue workers into the worst hit areas.

On the ground, officials are keeping virtually anybody but rescue teams out of the coastal communities flattened by the huge waves.

Police in devastated Yamashita have started the grim task of looking for bodies. They are going through the smashed houses and every time they find a new body, they bring a stretcher and they all bow and pray before taking the body out, placing it in a body bag, wrapping it up and then taking it away to a makeshift morgue.

Searchers have found 2,000 bodies in the Miyagi region. The local police chief said the death toll there was certain to exceed 10,000, with that many missing in the port town of Minamisanriku alone.

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

The authorities remain on high alert, with aftershocks regularly occurring and fears of more tsunamis.

On Monday a new tsunami scare triggered evacuations on the devastated north-east coast after a large wave was spotted rolling in to shore, but authorities later lifted an alert.

The United Nations said 590,000 people had been evacuated in the aftermath of the quake and tsunami disaster.

With ports, airports, highways and manufacturing plants shut down, the government has predicted "considerable impact on a wide range of our country's economic activities".

- ABC/wires