Two massive seismic jolts – similarly deadly, but as yet unrelated – brought chaos and destruction to a vast swath of the Pacific Rim yesterday, killing several hundred people and leaving thousands more feared trapped under piles of rubble on a day when nature showed her deadly hand.

A tsunami, set off by a powerful dawn earthquake, flattened villages on the Samoan islands and swept people and vehicles out to sea as surging water reached more than a mile inland. Just hours later, another large quake struck offshore from the Indonesian island of Sumatra, bringing down buildings and sparking widespread panic.

Last night, full details of the destruction caused by the ruptures deep beneath the Earth's crust were still emerging, but it was feared the combined death toll of about 300 would leap dramatically, possibly into the "thousands" as rescue workers scrambled into action across a massive area.

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It is believed 119 people died in Samoa, the neighbouring US territory of American Samoa and the country of Tonga, to the south, after the 8.3-magnitude quake triggered the Pacific's worst tsunami for more than a decade. One Briton, believed to be a two-year-old child, was among the dead, together with two Koreans, at least two Australians and a New Zealander.

Meanwhile, in American Samoa's capital, Pago Pago, the streets and fields were filled with ocean rubbish, debris and mud. In Washington, President Barack Obama declared a major disaster for the island and said that he and his wife, Michelle, "will keep those who have lost so much in our thoughts and prayers".

For all the destruction caused to the collection of Pacific archipelagos, it may have been the Indonesian island of Sumatra that suffered the highest human toll, with the city of Padang on the west coast bearing the brunt of the 7.6-magnitude quake.

There were reports that the roof of the main airport had caved in, as well as a shopping mall, two hospitals, schools and houses.

Indonesia's Health Minister, Siti Fadilah Supari, said the jolt was more powerful than a similar one three years ago that struck an historic city on the neighbouring Indonesian island of Java when more than 3,000 people died. "This is a high-scale disaster, more powerful than the earthquake in Yogyakarta," he said.

Rustam Pakaya, head of the health ministry's crisis centre, said that thousands of people were trapped under collapsed buildings.

"The earthquake was very strong," one survivor, Kasmiati, who lives on the coast near the quake's epicentre, told the Associated Press. "People ran to high ground. Houses and buildings were badly damaged. I was outside, so I am safe, but my children at home were injured."

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There were reports of the quake being felt in high buildings several hundred miles away in Jakarta, Singapore and Malaysia. In Padang itself, when the quake struck at 5.15pm local time, people sat down in the street to avoid falling over, such were the tremors – triggered along the same fault line that gave birth to the huge 2004 Asian tsunami that killed more than 230,000 people in a dozen countries.

The Indonesian Vice-President Jusuf Kalla said the death toll was likely to continue to rise sharply, as so many buildings had crumpled. "It's night-time now so it's dark," he said. "People are trapped, hotels have collapsed, schools have collapsed, houses have collapsed and electricity has been cut off."

Erwinsyah Sipahutar, a lecturer at the Padang Industrial Technic Academy, told an Indonesian website, Tempo, that people were "shaken like matchsticks". Footage broadcast by the local TVOne network showed children screaming as residents tried to put out fires started in the quake.

What will complicate today's rescue effort are the landslides that are reported to have cut off all roads to Padang. With power and communications also severed, those with relatives or friends in the city of 900,000 people were desperate for news.

"I want to know what happened to my sister and her husband," said Fitra Jaya, who owns a house in central Padang and was in Jakarta when the earthquake struck. "I tried to call my family there, but I could not reach anyone at all."

Killer quakes: 5 years of destruction

May 2008

The Great Sichuan Earthquake kills nearly 70,000 in China.

September 2007

Two earthquakes hit near Sumatra, Indonesia. The larger of the two, at a magnitude of 8.4, kills 25.

August 2007

A 8.0-magnitude earthquake strikes the Peruvian city of Ica, killing 650 and injuring 1,600.

April 2007

More than 28 are killed in the Solomon Islands after a tsunami is triggered by an earthquake measured at 8.1 on the Richter scale.

July 2006

A tsunami triggered by a 6.1- magnitude tremor near Java island, Indonesia, kills 600 people.

May 2006

Massive earthquake in Yogyakarta in the centre of Java leaves more than 5,000 dead.

October 2005

A great earthquake strikes Kashmir, killing 79,000 people in the Pakistan-administered region and more than 1,000 across the Line of Control in Indian-administered Kashmir.

March 2005

An earthquake hits Northern Sumatra, Indonesia, killing 1,300 people, most of them on the island of Nias.

December 2004

In the most catastrophic natural disaster of recent years, 230,000 people are killed by the Indian Ocean tsunami triggered by a huge 9.0-magnitude earthquake.