SENDAI, Japan (AFP) – A new explosion at a stricken nuclear power plant hit Japan Monday as it raced to avert a meltdown after a quake-tsunami disaster that is feared to have killed more than 10,000 people.

Searchers found 2,000 bodies in northeastern Miyagi region alone, while millions of Japanese were left without water, electricity, fuel or enough food. Hundreds of thousands more were homeless after the tsunami drowned whole towns.

Panic selling saw stocks close more than six percent lower on the Tokyo bourse on fears for the world's third-biggest economy, as power shortages prompted rolling blackouts and factory shutdowns in quake-hit areas.

Aid workers and search teams from across the world joined 100,000 Japanese soldiers in a massive relief push as the rattled country suffered a wave of major aftershocks and a false alarm over a new tsunami.

In hard-hit Ishinomaki, a town of about 165,000, it was a "desperate race against the clock to save those who may be trapped and wounded beneath colossal mounds of debris", said Red Cross Asia-Pacific spokesman Patrick Fuller.

"At the Red Cross hospital, no space is left unused. Exhausted Red Cross medics sleep side by side with the wounded," Fuller said in a Red Cross blog.

"And still droves of injured people in need of medical help arrive. The wounded arrive on foot, by helicopter or carried by their fellow citizens."

At least 1.4 million people in Japan are temporarily without running water and more than 500,000 people are taking shelter in evacuation centres, said the UN's Office for the Coordination of Humanitarian Affairs (OCHA).

"The main humanitarian needs are food, drinking water, blankets, fuel and medical items which the government and private sector in Japan are urgently mobilizing and sending to the affected area," OCHA said.

Rolling blackouts began across the nation in a bid to save power, with the heavily nuclear-dependent nation rocked by explosions and meltdown fears at its Fukushima power plant as well as an oil refinery fire.

The UN said power and gas supplies were critical, with the Japanese winter bringing sub-zero temperatures overnight and snow and rain forecast for coming days.

But it was the fear of a nuclear disaster looming on top of the quake and tsunami that gripped the embattled nation as it struggled with a crisis described by Prime Minister Naoto Kan as the worst since World War II.

Explosions have rocked two overheating reactors at the ageing Fukushima plant, located 250 kilometres (160 miles) northeast of Tokyo, after the cooling systems were knocked out by Friday's 8.9-magnitude quake.

A first explosion blew apart the building surrounding the plant's number-one reactor on Saturday but the seal around the reactor itself remained intact, officials said.

On Monday, shortly after Kan said the plant was still in an "alarming" state, a blast at its number-three reactor shook the facility, injuring 11 people and sending plumes of smoke billowing into the sky.

Chief government spokesman Yukio Edano said the plant's operator TEPCO reported the reactor was probably undamaged and there was a low possibility of a major radiation leak at the plant.

Later Monday the cooling system at the number two reactor failed, Jiji Press reported -- the sort of failure that preceded the explosions in the number one and three reactors.

Authorities have declared an exclusion zone within a 20 kilometre (12 mile) radius of the plant and evacuated 210,000 people.

A US aircraft carrier deployed off Japan for relief efforts has shifted its position after detecting low-level radiation from the malfunctioning Fukushima plant.

The ship was operating at sea about 160 kilometres (100 miles) northeast of the power plant at the time and a statement from the Seventh Fleet said the radiation level was so low that it presented no health risk.

As the nation struggled with the devastation wrought by the twin disasters of a shattered land and a surging sea, tsunami survivors who were able to outrun the killer wave recalled how they saw those behind them consumed by the torrent of mud and debris.

Miki Otomo's sister was one of the lucky ones, though the image of victims violently swept away last week by the black tide of wrecked houses and cars near the hard-hit city of Sendai will be forever seared in her memory.

"My older sister was in a bus when the wave came behind them. The bus driver told everybody to get out of the bus and run," said Otomo. "My sister was able to get away but some people just couldn't run fast enough," she told AFP.

Otomo, whose home near Sendai was destroyed, says she quickly piled her father and her dog in the car in her own desperate bid to survive.

"The tsunami wave was coming and I grabbed grandfather and our dog and drove. The wave was right behind me, but I had to keep zigzagging around obstacles and the water to get to safety."

Otomo is now staying at an evacuation centre in a local school with about 1,000 other exhausted survivors who cheated death.

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".

Leading risk analysis firm AIR Worldwide said the quake alone would exact an economic toll estimated at between $14.5 billion and $34.6 billion (10 billion to 25 billion euros), without taking into account the effects of the tsunami.

Kan said in a televised national address Sunday that Japan was facing its worst crisis since the end of World War II -- which left the defeated country in ruins after two US atomic attacks forced its surrender.

Japan sits on the "Pacific Ring of Fire", and Tokyo is in one of its most dangerous areas, where three continental plates are slowly grinding against each other, building up enormous seismic pressure.