Wellington  - More than 120 people died when a series of tsunamis triggered by a magnitude-8.0 earthquake swept across the South Pacific, reports from the region said.

At least 100 were killed in Samoa, the disaster management office of the island state of 220,000 people said. Several tourist resorts and villages were destroyed, it reported.

Another 19 people died in neighbouring American Samoa while 10 deaths were reported on Tonga's northern island of Niuatoputapu.

New Zealand's Stuff news website quoted witnesses as saying the devastation in Samoa was unimaginable "with bodies, covered in clothes, strewn around coastal villages."

Reports said cars and people, including children torn from the arms of their mothers, were swept out to sea by waves of up to 6 metres as survivors fled to high ground, where they remained huddled hours later.

New Zealand's acting Prime Minister Bill English told a news conference in Wellington there had been "considerable loss of life" with many more deaths likely to be reported in an unfolding tragedy.

An unknown number of New Zealand and Australian tourists, holidaying in Samoa, were believed to have died, trapped in tourist resorts as the tsunami followed one of the world's biggest earthquakes this year.

The south coast of Samoa's main island Upolu was devastated. "We've had very heavy damage all along the coast and most of the tourist resorts have been wiped out," Samoa's Deputy Prime Minister Misa Telefoni said.

Up to 20 people were reported killed in the beach resort Lalomanu, which is popular with New Zealand and Australian holidaymakers.

US President Barack Obama declared a "major disaster" in the US territory of American Samoa, freeing up federal funds for disaster relief.

English said a New Zealand air force maritime surveillance plane has been sent to look for survivors swept out to sea and other defence forces put on standby to fly medical aid and emergency shelters to Samoa, a former New Zealand territory.

The US Geological Survey put the magnitude of the quake, which was located about 204 kilometres south-west of Samoa, at 8.0 on the Richter scale after earlier estimating it at 8.3.

As aftershocks continued to hit the two Samoa states, seismologists reported two further quakes measuring 5.6 and a third of 5.8 in the South Pacific.

The Pacific Tsunami Warning Center lifted its alert to countries across the South Pacific about five hours after the first quake and people in Samoa were reported to be returning to the sites of their homes to inspect the damage and search for the missing.

Civil defence officials in New Zealand, 2,685 kilometres away from the quake's epicentre, issued an alert for the country's entire coastline after the Pacific Tsunami Warning Center in Hawaii said a 3-metre tsunami was travelling across the Pacific at about 800 kilometres an hour.

It lifted its warning after nine hours, telling people it was safe to return to the beaches. (dpa)