US navy helicopters began arriving in Banda Aceh, the devastated capital of Aceh province, on Saturday while a growing number of field hospitals were operating in the city and relief supplies poured into the local airport.

But the chief of the UN Office for the Coordination of Humanitarian Affairs in Indonesia, Michael Elmquist, told AFP that a myriad of coordination and infrastructure problems was making it impossible to quickly deliver the aid.

"It's going to take weeks before we get out to all the isolated areas," Elmquist told AFP by telephone from Jakarta after returning from Aceh's capital of Banda Aceh.

"It's a concern and we are desperately doing what we can to find ways of establishing distribution systems in Banda Aceh."

The UN and other relief organisations have warned people in Aceh's isolated areas, particularly in levelled towns along the west coast, may die within days if they do not immediately get clean water, food and medical treatment.

Asked if the delay was effectively a death sentence for some tsunami survivors, Elmquist replied: "I hope not. I hope that people are finding ways to get food and water, but obviously it's not a situation we are happy about.

"It's a very serious situation and we can only hope that it's possible to do something quicker."

The Indonesian government has said Sunday's tsunami may have already killed up to 100,000 people on Sumatra, although it has given up on delivering exact numbers because the death toll continues to rise so quickly.

Indonesian and international relief efforts have brought an enormous amount of aid to airports in Banda Aceh and Medan, the nearest major city in North Sumatra.

But with the floods destroying roads, bridges, telecommunication networks and almost all other basic infrastructure, distributing aid from the airports throughout Banda Aceh is proving a monumental task.

"At the moment the airport in Banda Aceh is being used to maximum capacity," Elmquist said.

"We are asking that anything that can be transported by truck from Medan should not be flown in."

The traffic jam at Medan airport was highlighted on Saturday when a Red Cross plane flying in from Paris with a water sanitation unit could not land because of the congestion and had to be re-routed to Kuala Lumpur.

Indonesian President Susilo Bambang Yudhoyono also expressed concern about the stagnant relief supplies during a tour of Aceh on Saturday, his second since the crisis.

"Immediately channel this aid," Yudhoyono told soldiers as he surveyed the backlog at Banda Aceh airport, according to the state Antara news agency.

"Do your duties as well as possible, day and night. We have the obligation to save each and every one."

As the relief operations continued, the people of Aceh experienced a series of aftershocks from the 9.0-magnitude earthquake that triggered the tsunamis.

One during the middle of the afternoon off Aceh's coast on Saturday measured between 6.4 and 7.0 on the Richter scale, according to various regional meteorological departments.

Other overnight aftershocks were recorded above 5.0 on the Richter scale, but although some were felt in Aceh, none caused any damage or led to another tsunami.

Meanwhile, Australian Prime Minister John Howard and his Japanese counterpart, Junichiro Koizumi, signed up for Indonesia's planned January 6 emergency summit of world leaders in Jakarta to discuss the crisis.

US Secretary of State Colin Powell and United Nations Secretary General Kofi Annan have also been slated as possible delegates, along with representatives from the 10 members of the Association of Southeast Asian Nations, Sri Lanka and India.