A huge aftershock rocked the Solomon Islands on Friday, sending villagers fleeing to higher ground two days after an 8.0-magnitude quake and tsunami which left 13 people dead and many more missing.
The 6.8-magnitude tremor was one of the strongest in a series of aftershocks that have been hampering relief efforts on remote Ndende island in the eastern Solomons, where Wednesday's tsunami inundated some 20 villages and left thousands homeless.
The quake, which was initially measured at 7.1 at a shallow depth of nine kilometres (5.5 miles) just 23 kilometres from the main town Lata, rocked buildings for 30 to 40 seconds, national disaster management office spokesman Sipuru Rove told AFP.
Rove, who was in the coastal town when the tremor hit, said villagers fled to higher ground.
Another major 7.0 tremor, followed by two smaller ones, hit in the early hours of Saturday but at a greater depth of 26 kilometres, the US Geological Survey said.
The aftershocks have prevented villagers from returning home and hampered their efforts to salvage supplies from damaged houses and bring them to makeshift camps in the rugged interior.
Planes attempting to fly aid to the area have also been wary of landing at the damaged airstrip in Lata amid the continuing quakes.
Geoscience Australia said that while Friday's tremor was smaller than the one that caused the destructive tsunami, the intensity of the shaking would have been much greater because it was much closer to the island of Ndende.
"I was more worried about the shaking aspect," he said. "This could have caused further issues," said seismologist David Jepsen.
A spokesman for Prime Minister Gordon Darcy Lilo's office, George Herming, said 13 people were so far confirmed dead and an undetermined number missing from the first quake, with the toll expected to rise as reports filter in from outlying areas.
Officials said damage on Ndende island was much worse than first thought.
"At first we thought it was going to be quite small but now it looks like it's going to be very big and communities will not be able to handle it themselves," national disaster management office spokesman Sipuru Rove told AFP.
"This is where we might require external assistance."
The prime minister's spokesman said the area was officially declared a disaster zone on Friday, as Australia announced it would assist relief efforts.
Australian Foreign Minister Bob Carr, who will visit the Solomons on Sunday, said the disaster "has been a devastating experience" for people in the low-lying islands.
In addition to the official toll, he said "many more are reported missing".
Carr announced 300,000 Australian dollars ($309,000) in relief payments and said a Royal Australian Air Force Hercules had been sent to the Solomons to provide logistical support.
Rove said the plane would make a reconnaissance flight over the island, giving officials their first comprehensive overview of the damage.
World Vision said food and water in the hillside camps was running low and sanitation would soon become an issue.
"Destruction has been widespread in and around Lata," it said. "Coastal wells have been covered by debris or contaminated, water tanks and toilets have been destroyed and coastal areas are littered with dead fish and poultry."
The US Geological Survey said Wednesday's quake struck in the middle of the day, beneath the sea about 76 kilometres west of Lata.
The Solomons are part of the "Ring of Fire", a zone of tectonic activity around the Pacific that is subject to frequent earthquakes and volcanic eruptions.
In 2007 a tsunami following an 8.0-magnitude earthquake killed at least 52 people in the Solomons and left thousands homeless.