On Nias, as many as 2000 people are feared to have died and many more were thought trapped under the rubble. A UN statement said 500 people were confirmed dead on the island.
Injured survivors pleaded for help two days after an 8.7-magnitude earthquake destroyed large parts of the island.
"Please sir, help us, we are starving," said a man in Gunungsitoli, the main town on Nias, as dozens of people looted a government store.
"They drive us away like pigs. We came here because we are hungry," said Abdul Murah Tanjung, 55, a Muslim resident of the mostly Christian island, outside the looted store.
Emergency aid on Wednesday began reaching the victims as Indonesian troops arrived to help in rescue and clearing efforts.
Indonesian troops have arrived
to help in the rescue and clean-up
Singaporean military helicopters were flying in food, water and medical supplies and evacuating the injured, witnesses said.
Officials said logistical problems were making it hard to help survivors on Nias, about 1400km northwest of Jakarta. About 700,000 live on the island.
In Sibolga on the nearby main island of Sumatra, Bill Sears, 65, a paediatrician and professor from California, was waiting to take a night ferry to Nias, accompanied by paramedics and a car full of medical supplies.
Sponsored by a private group, the team had been scheduled to go to tsunami-stricken Aceh but was diverted to Nias.
"There is an acute need for doctors over there. We're not sure what to expect. We just know there has been huge destruction," Sears said.
The United Nations and other international aid agencies - many already in the region because of the 26 December tsunami - have rushed to disaster-hit areas.
"There is an acute need for doctors over there. We're not sure what to expect. We just know there has been huge destruction"
The World Food Programme said it had deployed three helicopters and two aircraft.
The International Federation of Red Cross and Red Crescent Societies said a large landing craft carrying body bags, trucks and supplies was due to reach Nias shortly.
In Sibolga, an Indonesian naval ship was loading up with food and relief supplies.
Australian troops, with three Hercules aircraft and a ship, were due to deploy to the quake zone. Japan plans to send an 11-strong emergency medical team.