The U.S. Geological Survey says a magnitude 6.3 earthquake was reported off northern Sumatra's west coast, about 170 miles south of Banda Aceh.
There are no reports of casualties or damage, and no tsunami warning was put out.
Firefighters freed at least one survivor Wednesday. A man was found trapped in a crumpled house on remote Nias island, 36 hours after he was buried in rubble. As the first foreign military help arrived, officials said an estimated 1,000 people had died in the region's latest large earthquake.
Residents swarmed over collapsed buildings in Nias island's main town of Gunung Sitoli, searching frantically for survivors of the country's second catastrophe in three months, after December's massive quake and tidal wave.
There were occasional sounds of weeping as lifeless bodies were pulled from the debris, reports CBS News Correspondent Barry Petersen. Many were killed as they slept when the quake hit just before midnight.
"It's difficult to give an estimate, but we think that half of the buildings have collapsed, there are still people being stuck in those houses, and people are trying to get the survivors out," said Jeremy de Lage, an Oxfam coordinator who spent the night on the island.
The United Nations said Wednesday it had confirmed 518 deaths, but "That number is expected to rise," according to Masood Hyder, who is leading the United Nations relief operation in Banda Aceh.
French firefighters from the agency Firefighters Without Borders — who rushed to the island from Aceh province's west coast — used a car jack to free the legs of 25-year-old television repairman Jansen Silalalahi, who had been pinned between a motorbike and a cupboard.
As he was lifted out of the rubble of what was once a three-story building, Silalalahi smiled weakly and gave a thumbs-up.
"People knew I was there but it was difficult to reach me. I kept screaming whenever I heard anyone," said Silalalahi, who did not appear to be badly injured. "I feel relief because now I am safe."
The improvised rescue highlighted the crisis situation officials face here: there are thousands of collapsed buildings and no machinery to help search through the rubble for survivors.
There is virtually no medical care available on Nias, said de Lage.
"All the pharmacies are closed; we don't have access to medicine. The hospital is severely damaged. There is no medical staff there; only one doctor and two nurses," he said.
"We know there are many people critically injured," said Dr. Norman Peeler, a medical coordinator from the World Health Organization. "It is essential they get treatment, infections spread easily in open wounds."
Two Singaporean military helicopters landed Wednesday and distributed food and water to a frantic crowd of survivors. They also delivered a car, medical supplies, generators and 20 Singaporean troops and medics. A third helicopter was unable to touch down because there were so many survivors at the landing area.
Monday's 8.7-magnitude quake struck off Indonesia's Sumatra island, some 75 miles north of Nias. The even-bigger quake that generated the region's devastating tsunami on Dec. 26 hit an area further northwest along the Sumatran coast.
The latest quake initially raised fears of another tsunami and sent people scrambling for high ground in several Indian Ocean countries lashed by December's killer waves, but no big waves materialized.
However, it's not over for the people of Nias Island or anyone in the region, reports Petersen, as the aftershocks continue.
North Sumatra Gov. Rizal Nurdin estimated that 1,000 people died in the latest disaster, but officials feared the number could climb to 2,000. Bodies were still being discovered in the ruins of houses and shops on Wednesday and laid out in front of churches and mosques.
Looting broke out in at least one location on Nias when men, women and children scrabbled through a two-story store and left with boxes of noodles, clothes and a television set.
"There is no water, electricity or rice. Things are getting tough, we have had no help so what can we do?" asked Marzuki Tanjung, who was not among the looters.
Indonesia "welcomes and is open to all kind of assistance, including help from foreign troops, to assist in the disaster zone," Malarangeng told The Associated Press.
Australia and Japan were also planning military relief operations.
Japan said Wednesday it would send an 11-member emergency medical team and $140,000 worth of blankets, generators, sleeping pads and tents. Australia dispatched two military transport planes with medical supplies, and diverted a transport ship to the new disaster zone.
"We are providing today a lot of water and sanitation equipment, because there is a huge public health risk there because people don't have access to water. The city water supply has collapsed," said Oxfam's de Lage.
U.N. agencies were trying to coordinate deliveries of food, fresh water and medical supplies by helicopter. The agencies have stockpiles of supplies in the region to help feed and care for survivors of the Dec. 26 quake and tsunami, which killed more than 126,000 on Sumatra and left about half a million homeless.
Gunung Sitoli's main mosque was turned into a morgue for 21 Muslim victims. At a makeshift clinic outside the mosque, volunteers were running out of supplies.
"I have not slept since the earth began to shake," said Dr. Lucas Sapto, an Indonesian volunteer who was treating children with cuts on their faces.
A Chinese temple had about 20 bodies laid out in the tropical heat.
"We are waiting for a hearse. Once it comes, I can bury my daughter and two grandchildren," said Lukmin, a 74-year-old Chinese Indonesian Buddhist, who, like many in this country, uses only one name.
Nias appeared to have borne the brunt of the quake, but details of casualties on neighboring islands were sketchy.
"We have to be careful in counting" the dead, said Budi Atmaji Adiputro, chairman of Indonesia's Coordinating Agency for National Disaster Relief, adding, "We just have to count when we have seen the bodies."