Survivors dig with their bare hands, looking for food and water
Up to 10,000 US troops will be on the ground or off the coast of Haiti by Monday to help deal with the earthquake aid effort, US defence officials say.
Tuesday's earthquake has left as many as 50,000-100,000 people dead.
Haitian Prime Minister Jean-Max Bellerive said more than 15,000 bodies had already been recovered and buried, French news agency AFP reported.
The UN has launched a flash appeal for $562m (£346m), saying three million people would need help for six months.
Humanitarian chief John Holmes said a massive effort was being mounted and officials were "straining every nerve" to help.
US President Barack Obama described the scale of the devastation as extraordinary and the losses suffered as "heartbreaking".
AT THE SCENE
Matt Frei, Port-au-Prince
No-one is in charge. The president is sleeping at the airport with quite a few journalists and aid workers.
Earlier this morning, I stood on top of the rubble of the Supreme Court, the Foreign Ministry, the Interior Ministry and the Senate - where a few senators had been killed when the quake hit. Their bodies have been dragged out and put in body bags. The representatives of state are literally lying on the pavement slowly rotting away.
This is a citizenry left to its own extremely meagre resources. You've got ordinary people trying to administer IV drips to their family members who are slowly dying, but not a single doctor or nurse at the general hospital.
In a statement at the White House, he said the US would "do what it takes to save lives and help people get back on their feet".
Correspondents say survivors seem increasingly desperate and angry as bottlenecks and infrastructure damage delay relief efforts.
Many are spending another day without food and shelter in the ruined capital.
Mr Holmes earlier told reporters that 30% of buildings throughout Port-au-Prince had been damaged, with the figure at 50% in some areas.
The Pan American Health Organization has estimated that the death toll could be as high as 100,000.
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.
President Obama: "The scale of devastation is extraordinary... the losses heartbreaking"
The chairman of the US Joint Chiefs of Staff, Adm Mike Mullen, said a hospital ship and more helicopters would be sent in the coming days, carrying more troops and marines.
He said the total number of US troops would rise to between 9,000 and 10,000.
"Right now, I mean, literally as we speak, the Vinson (aircraft carrier) and the company from the 82nd Airborne who got there last night are focusing on delivering water from the helicopters offshore to the people of Haiti."
They want us to provide them with help, which is, of course, what we want to do
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.