Huge waves that witnesses and officials said measured between three and 7.5 metres high pounded the remote Pacific islands of Samoa and Western Samoa after an 8.0-magnitude undersea quake struck early Tuesday.

While the quake toppled buildings and sent thousands fleeing to high ground as the tsunami approached, many others were hit by the walls of water that swept people and cars out to sea and obliterated coastal settlements.

US President Barack Obama called the incident in the outlying US territory of American Samoa a "major disaster" and vowed "aggressive" action to help survivors.

"I am closely monitoring these tragic events, and have declared a major disaster for American Samoa, which will provide the tools necessary for a full, swift and aggressive response," he said.

"My deepest sympathies are with the families who lost loved ones and many people who have been affected by the earthquake and the tsunami," Obama added.

Samoa's Prime Minister Tuilaepa Sailele Malielegaoi said he was "shocked beyond belief."

"So much has gone. So many people are gone," he told the Australian news agency AAP. "I'm so shocked, so saddened by all the loss."

The tsunamis swept across the Pacific, battering Samoa where hospital workers said it killed at least 84 people, American Samoa where it felled 22, and Tonga, where at least seven people died.

As Australia, New Zealand and the United States led with immediate pledges of assistance, scores more people were missing feared dead in the chaos and despair that the twin disaster left in its wake.

Up to 70 villages stood in the way of the waves in the worst-hit area and each housed from 300-800 people, local journalist Jona Tuiletufuga told AFP.

Nine members of one family were killed in the village of Lalomanu on the south-east of Samoa, a relative said.

"My family own the Taufua Beach Fales and we have confirmation that nine members of our family have perished, four of them children and many more missing," the bereaved relative told Australia's public broadcaster.

"The tourists haven't been accounted for either."

Amateur video footage showed villages that had been completely obliterated, homes reduced to shards of metal and wood, while cars were stuck in treetops where they had been hurled by the force of the tsunami.

Samoa's deputy prime minister Misa Telefoni said his tiny country's tourism hotspot was "devastated" by the tsunami which left residents and holidaymakers with little time to flee.

"We've heard that most of the resorts are totally devastated on that side of the island. We've had a pretty grim picture painted of all that coast," he said.

Two of the country's most popular resorts, Sinalei Reef Resort and Coconuts Beach Resort, off the west coast of the main island of Upolu, had been hit hard, he told AAP.

Australia said at least three of its citizens, including a six-year-old girl, had died. Seoul said two Koreans were killed, while one person from New Zealand was also feared dead.

Apia, capital of the independent state of Samoa and nearly 3,000 kilometres (1,864 miles) from Auckland in New Zealand, was evacuated as officials scrambled to get thousands of residents to higher ground.

Officials in American Samoa, about 100 kilometres from Samoa, said the death toll of 22 was expected to climb.

"It could take a week or so before we know the full extent," Michael Sala, Homeland Security director in American Samoa, told AFP.

Waves around 25 feet (7.5-metres) high did most of the damage as they swept ashore about 20 minutes after the earthquake, demolishing buildings in coastal areas, he said.

Witnesses said cars were swept out to sea and buildings were destroyed in what the US territory's Congress delegate said was a scene of "devastation."

The eastern part of the island was without power and water supplies after the devastating earthquake, which struck at 6:48 am Tuesday (1748 GMT) at a depth of 18 kilometres (11 miles), 195 kilometres south of Apia.

The US Geological Survey (USGS) continued to report dozens of moderate aftershocks in the vicinity of the major quake, including a 5.2-magnitude quake that struck almost exactly 24 hours after the first.

The agency said the aftershock, which hit at 1747 GMT Wednesday, had been shallow -- 10 kilometres below the surface -- and was centred 255 kilometres southwest of Apia.

The United States, Australia and New Zealand all made preparations to send emergency help to the ravaged region that is home to more than 241,000 people.

The United Nations said Wednesday it was sending an emergency team to Samoa. The UN Office for the Coordination of Humanitarian Affairs was dispatching an emergency team to the Pacific territory and the UN system was mobilizing to provide aid, a statement said.

background report
What is a tsunami?
A once-exotic word that has now entered everyday use as a term tinged with fear, a tsunami refers to a shock of water propagated through the sea, usually after an undersea quake.

A section of seabed is thrust up or driven down by movement of Earth's crust.

The rift displaces vast quantities of water that move as waves, able to span enormous distances and sometimes with the speed of a jet plane.

The 8.0-magnitude quake that occurred under the Pacific Ocean near the Samoan islands on Tuesday unleashed waves estimated by witnesses and officials to reach 3.0 and 7.5 metres (10 and 24.3 feet) high. According to a still-incomplete toll, 113 people were killed.

When tsunamis approach a coastline, the shelving of the sea floor causes them to slow down -- but also gain in height.

To those on the shore, the first sign of something amiss is an eerie retreat of the sea, which is followed by the arrival of exceptional waves.

"The sea was driven back, and its waters flowed away to such an extent that the deep sea bed was laid bare and many kinds of sea creatures could be seen," wrote Roman historian Ammianus Marcellus, awed at a tsunami that struck the then-thriving port of Alexandria in 365 AD.

"Huge masses of water flowed back when least expected, and now overwhelmed and killed many thousands of people.... Some great ships were hurled by the fury of the waves onto the rooftops, and others were thrown up to two miles (three kilometres) from the shore."

Several factors determine the height and destructiveness of a tsunami.

They include the size of the quake, the volume of displaced water, the topography of the sea floor as the waves race to the coast, and whether there are natural obstacles that dampen the shock.

Destruction of protective mangroves and coral reefs, and the building of homes or hotels on exposed beaches, are fingered as leading causes of high death tolls from tsunamis.

Large quakes are the main drivers of tsunamis, but the phenomenon can also be sparked by other cataclysmic events, such as volcanic eruptions and even landslides.

In 1883, a volcano shattered the Pacific island of Krakatoa, causing a blast so loud that it could be heard 4,500 kilometres (2,800 miles) away, followed by a tsunami that killed some 30,000 people.

The great tsunami of December 2004 in the Indian Ocean was caused by a monstrous 9.1-magnitude earthquake off the Indonesian island of Sumatra.

It released the energy of 23,000 Hiroshima-type atomic bombs, according to the US Geological Survey (USGS). Some 220,000 people in 11 nations were killed, many of them thousands of kilometres from the epicentre.

The Pacific Ocean is particularly prone to earthquakes and therefore to tsunamis -- indeed the word "tsunami" comes from the Japanese words for "harbour" and "wave".

But recent research has found that, over the millennia, tsunamis have occurred in many parts of the world, including the Atlantic and Mediterranean. A global monitoring network, overseen by the UN, has been set in place to alert areas at risk.

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