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.
Leaving for Haiti, UN Secretary General Ban Ki-moon said the quake was the worst humanitarian crisis for decades.
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
Nick Davis, BBC News, Haiti
Relief is finally getting through to some in Port-au-Prince but it's a trickle - not a flood - of the aid needed by the people here.
The US navy is using helicopters to drop supplies of bottled water using soldiers on the ground to keep control. The UN also has distribution points handing out high-energy bars to the hungry.
But demand is outstripping supply - with food and water being taken faster than they can pass it out.
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 a woman alive from 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.
Airport 'overwhelmed'
Correspondents say although the amount of supplies getting through is still small, there is a sense of movement at last.
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.
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."
'No help'
The UN says up to 80-90% of buildings in Leogane, about 19km west of Port-au-Prince, have been destroyed.
map
Satellite and close-up images of Port-au-Prince devastation
One survivor in the town said he had come to Haiti from America for his mother's funeral, only for his wife to be killed in the earthquake. He said that so far people in the area had received no help of any kind.
"We don't have any aid, nothing at all," he said. "No food, no water, no medical, no doctors."
Estimates of how many people died following the 7.0 magnitude earthquake on Tuesday have varied.
The Pan American Health Organization put the death toll at 50,000-100,000, while Haitian Prime Minister Jean-Max Bellerive said 100,000 "would seem a minimum".
A UN official has said aid workers are dealing with a disaster "like no other" in UN memory because the country had been "decapitated".
Three ministers and several senators are reported to have been killed.
The US has launched what President Barack Obama called "one of the largest relief efforts in its history" following the quake.
US Secretary of State Hillary Clinton was the first senior Western official to arrive in Haiti, on Saturday.
She told Haitians that the US would be "here today, tomorrow and for the time ahead", asserting that "Haiti can come back even better and stronger in the future".
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