Vietnam, Cambodia and Laos were the latest countries to be lashed by Typhoon Ketsana as it continued a rampage that began in the Philippines, killing over 330 people and forcing millions to flee their submerged homes.

As outside powers geared up to help, disaster struck again further east when a powerful 8.0-magnitude undersea quake unleashed tsunamis on the vulnerable Pacific islands of Samoa, American Samoa and Tonga.

At least 113 people were killed, including foreigners from Australia, Britain and South Korea, as waves 25 feet (7.5 metres) high wiped out villages, flattened tourist resorts and sent people scurrying for high ground.

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.

US President Barack Obama called the incident in the outlying US territory of American Samoa a "major disaster" and offered Samoans his "deepest sympathies".

He also dispatched troops for the aid effort in the Philippines, a former American colony.

With the Philippines reeling from once-in-a-lifetime floods that have inundated Manila, officials in the mainly Catholic country urged people to pray for deliverance from a new menace lurking to the east, Typhoon Parma.

Yet more disaster hit in Indonesia Wednesday, when a 7.6-magnitude quake rocked the island of Sumatra, killing at least 75 people and trapping thousands under rubble.

Large buildings including hospitals and hotels caved in, while fires raged in the coastal city of Padang, home to nearly a million people, and outside rescuers struggled to reach the scene.

The early death toll looked set to rise dramatically, said officials.

"Maybe more than 1,000... because so many buildings and houses have been damaged," said Health Ministry Crisis Centre head Rustam Pakaya

Rescue teams and doctors sent overland were expected in the city on Thursday morning, Pakaya said.

Frightened office workers streamed out into the streets as tremors were felt in Jakarta, 940 kilometres (580 miles) away, and in the Malaysian capital Kuala Lumpur.

As Washington grappled with a situation in American Samoa that evoked memories of Hurricane Katrina, the string of Asia-Pacific disasters highlighted pitiful defences in some nations and underlying worries about global warming.

In flood-hit Vietnam, the head of the Red Cross in the city of Danang, Phan Nhu Nghia, described aid efforts as "very, very difficult, even with a greater mobilisation from the soldiers and the police, because the scale of the flooding is too vast and we lack equipment".

"We have not received any support from local authorities," a 28-year-old mother of twin toddlers complained to AFP in Vietnam's Quang Nam province.

She described local residents having to wade into flood waters in search of clean water and supplies of instant noodles. "My house has been flooded since last night," said the woman, huddling with a group of women, children and the elderly.

Away from the immediate mayhem, the Philippines' chief negotiator at climate talks in Bangkok, Heherson Alvarez, said he hoped Ketsana had driven home "the sense of urgency" in global talks to alleviate climate change.

"Tropical storm Ketsana is clearly a manifestation of the consequences of global inaction in addressing the immediate impacts of creeping climate change," he 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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