The devastating tsunami triggered by the biggest earthquake on record in Japan has destroyed huge areas of the country's northeastern coast.

The 8.9 magnitude quake also prompted fears for the safety of Japan's many nuclear reactors. Thousands of residents have been evacuated from an area around Fukushima nuclear plant after pressure and radiation levels rose in the reactor.

Authorities are planning to vent pressure in the reactor, a move which may release an amount of radioactive material into the atmosphere.

Underscoring grave concerns about the Fukushima plant - some 240km north of Tokyo - US Secretary of State Hillary Clinton said the US air force had delivered coolant to avert a catastrophic rise in the temperature of the facility's nuclear rods.

The quake, the most powerful since Japan started keeping records 140 years ago, has sparked at least 80 fires in cities and towns along the coast.

Other Japanese nuclear power plants and oil refineries have been shut down and one refinery remains ablaze. Kyodo news agency has reported that a ship with 100 passengers and a passenger train are still unaccounted for after the tsunami.

The first quake struck just 400km northeast of Tokyo, the US Geological Survey said. It was followed by more than 40 aftershocks, one as strong as 7.1.

Stunning TV footage showed a muddy torrent of water carrying cars and wrecked homes at up to 500km/h across farmland near the coastal city of Sendai, home to one million people.

Ships, boats, cars and trucks were tossed around like toys in the water after thel tsunami hit the town of Kamaichi in northern Japan.

Thousands remain missing and the extent of the destruction along a lengthy stretch of coastline suggests the death toll could rise steeply.

Tsunami warnings were earlier issued across the Pacific but were later lifted for some of the most populated countries in the region, including Australia, Taiwan and New Zealand.

The unfolding disaster has prompted offers of help from dozens of countries.

China said rescuers were ready to help with quake relief while President Barack Obama told Japanese Prime Minister Naoto Kan the United State would assist in any way it could.

Even in a nation accustomed to earthquakes, the devastation in Japan 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.

Japan has prided itself on its speedy tsunami warning system, which has been upgraded several times since its inception in 1952.

The country has also built countless breakwaters and floodgates to protect ports and coastal areas, although experts said they might not have been enough to prevent disasters as unprecedented as what occurred this morning.

The Department of Foreign Affairs says concerned relatives of Irish citizens should contact it on 01-4082000.

They can also register details of Irish people in Japan on the website