Search and rescue teams have begun to arrive on the Indonesian island of Nias, which was worst hit by Monday's massive earthquake.
Damage to the infrastructure is hampering the aid effort
Damaged infrastructure and bad weather are hindering rescue efforts.
Nias residents have been scrambling for aid supplies and searching for survivors. The main town, Gunung Sitoli, has been largely destroyed.
The UN has confirmed 518 were killed in the 8.7-magnitude tremor, but it is thought up to 1,000 may have died.
Monday's quake caused panic across the Indian Ocean region, which is still reeling from the massive waves that killed some 300,000 people in December.
Indonesia was home to two-thirds of those killed in December.
Night has fallen in Nias and many survivors are now sleeping in tents outside, fearing the possibility of another tremor.
Damage to the tiny airport in Gunung Sitoli means that only light aircraft and helicopters are able to land, slowing the delivery of aid.
More is being sent by sea, but that will take much longer to get there.
Norwegian, French and Spanish search and rescue teams have arrived from the west coast of Aceh, where they have been helping survivors of December's tsunami.
The priority now is to try to reach survivors who may be trapped under the rubble. But food, water, tents and supplies of fuel to power generators are also urgently needed.
A lack of heavy lifting equipment has left residents searching for survivors using basic tools and their bare hands.
Many of those feared dead are children who were sleeping when the earthquake struck.
The governments of Australia, Japan and Malaysia have promised to send troops, relief teams and supplies.
An official from the UN agency co-ordinating relief for the island said the weather delayed helicopters ferrying machinery and supplies from the Sumatra port of Sibolga to Nias for several hours.
"We were able to get some medical and rescue teams with equipment over to Nias. But the weather was not in our favour and one of our planes didn't show. We're hoping we'll get more in tomorrow," Michele Lipner told the AFP news agency.
Oxfam has also flown in basic emergency supplies including tents, food and water, body bags, fuel and basic medical supplies.
More specialist aid equipment including water pumps, water tanks and communications equipment is expected to be flown in on Friday, the charity said.
"This morning we managed to evacuate people by helicopter who had been critically injured in the quake. There are many others injured on the island who need urgent assistance," said Alessandra Vilas Boas, a member of the Oxfam assessment team.
Pleading for help
There was already a large international aid presence in the region following the earlier disaster. The BBC's Rachel Harvey in Aceh province - the region worst hit in December - says lessons appear to have been learned.
But relief officials say the situation remains critical.
The UN's deputy humanitarian co-ordinator in Banda Aceh, Masood Hyder, told AFP nine people had died on the neighbouring island of Simeulue, while nine were killed in an area which includes part of mainland Sumatra and the smaller Banyak islands.
But unconfirmed Indonesian reports suggest up to 300 people died on the Banyak islands.
Rescuers managed to save a 25-year-old man on Wednesday - three days after the quake.
Jansen Silalalahi was pinned between a motorbike and a cupboard until French firefighters, who had relocated from Aceh, managed to free him.
"There is a huge public health risk there, because the city water supply has collapsed, " Oxfam's Jeremy Delage told the BBC.
Looting has been reported in parts of the city.
"Please sir, help us, we are starving," said a man as dozens of people looted a store with police looking on, Reuters news agency reported.
Nias Island: Severely injured evacuated to Medan, Meulaboh and Sibolga. WHO sending medical teams from Meulaboh, UNHCR sending 500 tents, Unicef sending two water treatment plants, Oxfam sending tents, supplies, water and communications equipment
Simeulue: Aid agencies distributing tented hospitals, emergency rations and communications equipment
Banyak Islands: UN team has made an aerial survey only. Unconfirmed Indonesian government reports say 300 people could have died
Other developments: Singapore government and the Red Cross sending humanitarian teams to the region
Source: UN OCHA situation report, 29 March 2005, Oxfam