US Secretary of State Hillary Clinton has become the first senior Western official to arrive in Haiti after the earthquake that flattened the capital, BBC reported.
Mrs Clinton said she was visiting to express the US's "long-term, unwavering support, solidarity and sympathies".
Tens of thousands of people were killed and survivors have grown desperate as they wait for aid to arrive.
Relief has been arriving, but little has moved beyond the jammed airport and there are reports of gangs and looting.
There is little police presence in Port-au-Prince - a BBC correspondent was able to count six throughout the day on Friday - although some Brazilian UN peacekeepers are patrolling the streets.
On Saturday morning, a magnitude-4.5 aftershock struck close to Haiti's capital, the US Geological Survey said, forcing people to flee buildings.
According to Haitian Interior Minister Paul Antoine Bien-Aime, 50,000 bodies have been collected, but the total number of dead could be "between 100,000 and 200,000".
Damage to the seaport, roads and other infrastructure has prevented the speedy distribution of supplies.
US President Barack Obama met two of his predecessors in the White House - George W Bush and Bill Clinton - to seek their support.
After the talks, Mr Obama said the two men would lead the US fundraising efforts through the Bush-Clinton Haiti Fund.
"America is moving forward with one of the largest relief efforts in its history," Mr Obama said, warning that recovery would take a long time.
President Bush urged Americans to send "cash", and President Clinton said Haitians "can escape their history and built a better future if we do our part".
A spokeswoman for the UN Office for the Co-ordination of Humanitarian Affairs said aid workers were dealing with a disaster "like no other" in UN memory because the country had been "decapitated".
"Government buildings have collapsed and we do not even have the support of the local infrastructure," Elisabeth Byrs said in Geneva.
Ms Byrs said the situation was even worse than the devastation wrought by the 2004 tsunami in Indonesia's Aceh province.
"It's worse than the Indonesian earthquake where at least we could get the support of some local authorities," she said.
The UN has launched an appeal for $562m (£346m) intended to help three million people for six months.
A total of about $360m has been pledged so far for the relief effort, but only part of this sum will be included in the emergency appeal.
US authorities have taken temporary control of the airport to help distribute aid more quickly.
Aid may be arriving in huge quantities but there is little of it to be seen in Port-au-Prince, says the BBC's Nick Davies in the capital.
And many people continue to leave the city, in search of food, water and medicine.
The UN is reporting a rise in the number of people trying to cross into the neighbouring Dominican Republic, and an influx into Haiti's northern cities.
The US has already sent an aircraft carrier, the USS Carl Vinson, to Haiti and the USS Bataan, carrying a marine expeditionary unit, is on its way.
A hospital ship and more helicopters are due to be sent in the coming days, carrying more troops and marines, with the total number of US troops to rise to between 9,000 and 10,000.
Aid groups say it is a race against time to find any more trapped survivors.
Plane-loads of rescuers and relief supplies have arrived or are due from the UK, China, the EU, Canada, Russia and Latin American nations.
The UN said about 300,000 people had been made homeless.
Meanwhile, details are emerging about the extent of the damage beyond Port-au-Prince. Up to 90% of the buildings have been damaged in Leogane, a town about 19km (18 miles) to the west, the UN said.
"According to the local police, between 5,000 to 10,000 people have been killed and most bodies are still in the collapsed buildings," Elisabeth Byrs said.