Emergency supplies are on their way to the Indonesian island of Nias, where more than 1,000 people are feared dead after Monday's massive earthquake.
Medical help is on its way to the survivors
The 8.7 magnitude quake devastated the island, demolishing buildings in the main town, Gunung Sitoli.
Aid workers from UK charity Oxfam say the town's 20,000 people are without water and roads have collapsed.
The quake caused panic across a region still reeling from the 26 December tsunami that killed up to 300,000.
The islands of Nias and Simeulue bore the brunt of the quake, which struck at around 2315 local time (1615 GMT) and lasted up to three minutes.
Efforts to rescue and evacuate survivors are under way, with two Indonesian naval ships carrying medical supplies heading for the islands.
A field hospital is being prepared on the mainland to treat the injured. Food, tents and other basic supplies are being made ready for transport by boat and by air.
The response to this latest disaster has been swift and so far well co-ordinated, says the BBC's Rachel Harvey in northern Sumatra.
There was already a huge international aid presence in the region because of December's devastating earthquake and tsunami, our correspondent adds.
Reports quoting Indonesian officials suggest at least 400 people were killed.
But Indonesia's Vice-President Jusuf Kalla told the BBC "more than 1,000 people were possibly dead" in Nias alone.
Around 80% of buildings had been affected in Gunung Sitoli, Mr Kalla said.
The Indonesian Red Cross also issued a statement saying more than 1,000 were estimated to have died on Nias island.
Binahati Bahea, district chief of Gunung Sitoli, told the BBC the number of dead "will rise, because we have not managed to evacuate many buildings and there are many dead people there".
"Our problem at the moment is transportation because many bridges are damaged and it's difficult to reach many towns," he said.
Oxfam staff who went to a mosque in the town being used as a morgue and temporary shelter said 34 bodies had already been brought there.
Alessandra Villas-Boas, a member of the Oxfam team, said: "The water system has failed completely and huge holes have been made in the roads. Bodies are being pulled from the rubble as I speak."
She said Oxfam would begin flying in emergency supplies on Wednesday, including generators to provide power for the pumps operating the water system.
One student who survived the earthquake on Nias told AFP how his arm and fingers were broken during the ordeal.
Serasi Hulu, 20, said: "I was fast asleep when the earthquake occurred, but I woke up just in time to escape from the crumbling roof of my dormitory.
"Before I managed to get out of the house, part of the roof fell on me and I was trapped for several hours, along with two of my high school mates. I believe they may already have died."
President Susilo Bambang Yudhoyono has declared a state of emergency and is to travel to Nias on Wednesday.
Despite Nias's remoteness, it has become a popular resort in recent years with surfers and the money raised from tourism funded a mini building boom, says the BBC's Tim Johnston in Jakarta.
Reports say it was many of these buildings, made from concrete rather than more flexible traditional materials, that collapsed.
The epicentre of the latest earthquake was about 160km (100 miles) from that of the quake which caused last December's disaster.
However, no tsunami materialised on this occasion as a much smaller area of the ocean floor moved, seismologists say.
Monday night's quake was felt as far away as Malaysia and Singapore, and thousands fled their homes across the region.
But three hours after the quake, tsunami alerts were called off, and people began returning home.