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The quake hit at 6.48am local time midway between the two island groups. In Apia, families reported shaking that lasted for up to three minutes. The US Geological Survey said the quake struck 20 miles (35 km) below the ocean floor, 120 miles from American Samoa and 125 miles from Samoa.
The Pacific Tsunami Warning Centre in Honolulu, Hawaii issued a general alert for the South Pacific region, from American Samoa to New Zealand. It said there were indications a tsunami wave could be "destructive" along some coastlines.
The centre issued a tsunami warning for numerous islands in the Pacific, including the Samoas, the Cook Islands, Tonga, Fiji, New Zealand, French Polynesia and Palmyra Island.
A tsunami watch was issued for Hawaii, Vanuatu, the Marshall Islands, Solomon Island, Johnston Island, New Caledonia, Papua New Guinea, Wake Island, Midway Island and Pitcairn.
New Zealander Graeme Ansell said the beach village of Sau Sau Beach Fale was levelled.
"It was very quick. The whole village has been wiped out," Ansell told New Zealand's National Radio from a hill near Samoa's capital, Apia. "There's not a building standing. We've all clambered up hills, and one of our party has a broken leg. There will be people in a great lot of need round here."
A tsunami swept into Pago Pago, capital of American Samoa, shortly after the earthquake, sending sea water surging inland about 91 metres (100 yards) before receding, leaving some cars stuck in mud.
The staff of the port ran to higher ground, and police soon came by, telling residents to get inland.
In Fagatogo, water reached the waterfront town's meeting field and covered portions of the main highway, which also was plagued by rock slides.
In Samoa, the powerful quake jolted people awake.
"It was pretty strong; it was long and lasted at least two minutes," one resident told local radio.
"It's the strongest I have felt, and we ran outside. You could see all the trees and houses were shaking," he said.
Sulili Dusi told National Radio that "everything dropped on the floor and we thought the house was going to go down as well. Thank God, it didn't". Along with neighbours, they fled to high ground.
She said the tsunami hit the south side of the island, and some "cars have been taken". She did not elaborate, but added "we just thank God no life has been taken yet".
Another resident, Dean Phillips, said the southern coast of Upolu island had been struck by the tsunami.
"The police are sending everybody up to high ground," he said.
Local media said they had reports of some landslides in the Solosolo region of the main Samoan island of Upolu and damage to plantations in the countryside outside Apia.
There were no immediate reports of injury or serious damage from local emergency services, but people reported cracks in some homes and items tossed from shelves.