Tsunami Strikes Solomon Islands
Tsunami warnings have also been issued for Papua New Guinea, north-east Australia, and other nearby islands.
The quake measured 8.0 and hit at 0740 local time on Monday (2040 GMT Sunday).
It struck 345km (215 miles) north-west of the Solomon Islands' capital Honiara, north-east of Australia, the US Geological Survey said, at a depth of 10km (six miles) below the surface.
Evacuation
One of the worst affected areas is the town of Gizo, in the west of the Solomons, which was only 45km (25 miles) from the epicentre of the quake.
"There was 10ft of water rushing through town."
Harry Wickham
Hotel worker, Gizo
"All the houses near the sea were flattened," as water "right up to your head" swept through the town, said Gizo resident Judith Kennedy.
"A lot of houses have collapsed. The whole town is still shaking," she told the Associated Press.
Harry Wickham, a hotel worker in Gizo, told New Zealand television that "there was 10ft of water rushing through town".
There are reports that a local hospital has been inundated by water and at least three people are confirmed dead in the town.
Britain's deputy high commissioner in Honiara told the BBC that four people were also missing from Mono Island.
The earthquake was also felt in the capital, Honiara. David Leeming, who lives in the city, told the BBC News website that his five-storey office building shook and staff left the building for safety.
The Pacific Tsunami Warning Center initially issued a warning for all countries in the South Pacific region.
Later it was down-scaled, but authorities in the Solomon Islands, Papua New Guinea, Vanuatu, New Caledonia, north-eastern Australia, Tuvalu, Kiribati, and Fiji were being advised to stay on alert.
Officials in Australia said the coast of New South Wales could face dangerous waves and flooding in the coming hours.
Several beaches in Sydney have been closed as a precaution.
Experts in Japan and Hawaii were also monitoring the situation.