Underscoring grave concerns about the plant, the US air force delivered coolant to the facility, US Secretary of State Hillary Clinton said. The unfolding disaster in the wake of the 8.9 magnitude earthquake and 10-metre (33-feet) high tsunami prompted offers of help from dozens of countries.

China said rescuers were ready to help with quake relief while US President Barack Obama told Japanese Prime Minister Naoto Kan the United State would assist in any way. Stunning TV footage showed a muddy torrent of water carrying cars and wrecked homes at high speed across farmland near the coastal city of Sendai, home to one million people and which lies 300 km (180 miles) northeast of Tokyo. Ships had been flung onto a harbour wharf, where they lay helplessly on their side.

Japanese politicians pushed for an emergency budget to fund relief efforts after Kan asked them to "save the country", Kyodo news agency reported. Japan is already the most heavily indebted major economy in the world, meaning any funding efforts would be closely scrutinised by financial markets.

Domestic media said the death toll was expected to exceed 1,000, most of whom appeared to have drowned.

The extent of the destruction along a lengthy stretch of coastline suggested the death toll could rise significantly. Tsunami warnings were issued across the Pacific but were later lifted for some of the most populated countries in the region, including Australia, Taiwan and New Zealand.

Even in a nation accustomed to earthquakes, the devastation was shocking.

"A big area of Sendai city near the coast, is flooded. We are hearing that people who were evacuated are stranded," said Rie Sugimoto, a reporter for NHK television in Sendai. "Around 140 persons, including children, were rushed to an elementary school and are on the rooftop but they are surrounded by water and have nowhere else to go." The quake, the most powerful since Japan started keeping records 140 years ago, sparked at least 80 fires in cities and towns along the coast, Kyodo said.

Other Japanese nuclear power plants and oil refineries were shut down and one refinery was ablaze. Television footage showed an intense fire in the waterfront area near Sendai.

There were also reports that an irrigation dam had broken and swept away houses in Fukushima prefecture.

Chief Cabinet Secretary Edano told people to stay in safe places as the cold deepened into the night. "Please help each other and act calmly," he told a news conference. In Tokyo, residents who had earlier fled swaying buildings jammed the streets trying to make their way home after much of the city's public transportation was halted. Many subways in Tokyo later resumed operation but trains did not run. People who decided not to walk home slept in office buildings. "I was unable stay on my feet because of the violent shaking. The aftershocks gave us no reprieve. Then the tsunamis came when we tried to run for cover. It was the strongest quake I experienced," a woman with a baby on her back told television in northern Japan.

The central bank said it would cut short a two-day policy review scheduled for next week to one day on Monday and promised to do its utmost to ensure financial market stability.