UN and Oxfam staff are finally bringing food and water to some parts of the capital Port-au-Prince, but the airport remains clogged with loaded planes.
Many survivors of Tuesday’s quake have become desperate as they wait for aid, and many are trying to leave the city.
UN Secretary General Ban Ki-moon, who has arrived in Haiti, said it was the worst humanitarian crisis for decades.
Mr Ban is expected to visit the ruins of the UN mission, where several staff including Special Representative Hedi Annabi were killed, and meet President Rene Preval.
The UN has launched an appeal for $562m (£346m) intended to help three million people for six months, while some two million people are thought to need emergency relief.
AT THE SCENE
Nick Davis, BBC News, Haiti
In this highly Christian country, there are still songs of praise being sung, but also prayers for the dead and dying.
Relief has started trickling through, with NGOs, the UN and the US military getting food and water to those who need it.
But demand has so far outstripped supply, as the distribution points become quickly overwhelmed by the numbers of people clamouring for supplies.
Bottlenecks still exist between getting the aid, which is arriving in large amounts, to the people who desperately need it.
Meanwhile first reports from the epicentre of the earthquake suggest the damage is even more dramatic than in the capital.
The BBC’s Mark Doyle in Leogane, west of Port-au-Prince, described the scene as “apocalyptic”, with thousands left homeless and almost every building destroyed.
But in a sign of hope, rescuers pulled three people alive from the rubble on Sunday. Twelve others were rescued on Saturday, the UN said.
There are also security concerns amid reports of looting.
The US Southern Command’s Lt-Gen Ken Keen said that while streets were largely calm there had been an increase in violence.
“We are going to have to address the situation of security,” he said, quoted by the Associated Press.
“We’ve had incidents of violence that impede our ability to support the government of Haiti and answer the challenges that this country faces.”
AFP news agency quoted one of its photographers as saying police had opened fire on looters in a Port-au-Prince market, killing at least one of them.
Correspondents say although the amount of supplies getting through is still small, there is a sense of movement at last.
UN LOSSES IN HAITI
37 UN staff confirmed dead, more than 300 missing
Includes Special Representative Hedi Annabi, deputy Luiz Carlos da Costa and acting police commissioner Doug Coates
UN HQ in the Christopher Hotel and other buildings collapsed in the quake
Believed to be the biggest single loss of life in the UN’s history
The UN World Food Programme has been handing out aid packages containing food, while UK charity Oxfam has been distributing water.
US troops said they had set up their first foothold outside the airport to deliver aid carried in by helicopters.
But many victims are still not receiving any aid, as the airport remains a bottleneck. UN Humanitarian Coordinator Kim Bolduc says getting supplies out to them from the planes is still a major hurdle.
“The Haitian airport now is overwhelmed,” said UN Assistant Secretary General for Peacekeeping Operations, Edmond Mulet.
The port is badly damaged, and many roads still blocked by corpses and debris.
David Wimhurst, a spokesman for the UN peacekeeping force in Haiti, said aid was being delivered as quickly as possible.
“Aid is going out but it’s simply impossible in 24 hours to bring in enough aid to instantly feed all these people, many of whom are in places that are inaccessible,” he said.
We need fuel to bring in supplies and carry the wounded
The Haitian and Dominican Republic governments are planning an alternative 130km (80 miles) humanitarian road corridor to deliver relief supplies from the southern Dominican town of Barahona, the UN reports.
The UN has warned about fuel shortages, which it says could affect humanitarian operations.
“Fuel is the key issue,” Elisabeth Byrs, a spokeswoman for the UN Office for the Co-ordination of Humanitarian Affairs, told the BBC. “We need fuel to bring in supplies and carry the wounded.”
The UN says up to 80-90% of buildings in Leogane, about 19km west of Port-au-Prince, have been destroyed.