Survivors dig with their bare hands, looking for food and water
The UN has launched an appeal for $562m (£346m), to help victims of Tuesday's devastating earthquake in Haiti.
UN humanitarian chief John Holmes said the funds were intended to help three million people for six months.
The earthquake has left tens of thousands of people dead, and rescuers are continuing an increasingly desperate search for survivors.
Meanwhile, US Secretary of State Hillary Clinton said she would travel to Haiti on Saturday.
The BBC's Matthew Price, outside the ruins of a nursing college in the capital Port-au-Prince, says he has been told by a female member of staff that there could be 260 dead bodies and up to 25 people still alive under the rubble.
AT THE SCENE
Matthew Price, Port-au-Prince
Behind me is what remains of a five-storey building, a college of nursing that is now the size of a one-storey building.
According to one of the members of management here, inside there were believed to have been more than 200 people who were crushed when that building came down. It is also believed that there are possibly 25 people alive in there.
The reason for believing that is that last night a text message was received from someone who said they were inside, that they were very hungry, that they were very hot indeed and that they needed someone to come and rescue them.
In the last few hours a Brazilian search and rescue team has been very, very slowly trying to get access inside. Their truck has now started moving away.
A team of Brazilian rescuers is trying to gain access to the victims but progress is painfully slow, our correspondent adds.
Haitian Interior Minister Paul Antoine Bien-Aime told Reuters news agency that 50,000 bodies had already been collected.
"We anticipate there will be between 100,000 and 200,000 dead in total, although we will never know the exact number," he said.
The Pan American Health Organization has estimated that the death toll could be as high as 100,000, while the UN said about 300,000 had been made homeless.
The US has announced it will grant leave to remain to thousands of illegal Haitian migrants living there due to the humanitarian crisis in their country.
The chief of the homeland security department, Janet Napolitano, said they would be allowed to stay and work, initially for 18 months.
Despair and anger
Aid workers have been grappling with logistical problems as they attempt to distribute aid.
The port is too damaged to use and roads are blocked by debris, although the main route from the Dominican Republic is now clear.
This is a huge and a horrifying catastrophe, the full consequences of which we do not know
At the country's main airport, which is small and has been filled to capacity, US authorities took temporary control to help distribute aid more quickly.
Correspondents say survivors seem increasingly desperate and angry as bottlenecks and infrastructure damage delay relief efforts.
Mr Holmes, who heads the UN Office for the Coordination of Humanitarian Affairs (Ocha), said a massive effort was being mounted and officials were "straining every nerve" to help.
"This is a huge and a horrifying catastrophe, the full consequences of which we do not know," he said.
He said almost half of the appeal money would be for emergency food aid, with amounts of between $20m and $50m for health, water and sanitation, nutrition, emergency shelter, early recovery and agriculture.
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.
Mr Holmes earlier told reporters that 30% of buildings throughout Port-au-Prince had been damaged, with the figure at 50% in some areas.
Many there have spent a third day without food and shelter in the ruined capital, though UN Secretary General Ban Ki-moon, who is due to visit Haiti on Sunday, said distribution of food and medicine was under way.
The BBC's Nick Davis in Port-au-Prince says the only convoys he has seen are people leaving the city, in search of food, water and medicine.
Correspondents say there is little official presence in Port-au-Prince despite incidents of looting.
Mrs Clinton said she would visit Haiti on Saturday to assess the damage, meet government officials and convey to the Haitian people "our long term, unwavering support, solidarity and sympathies".
Earlier US President Barack Obama described the scale of the devastation as extraordinary and the losses suffered as "heartbreaking".
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".
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 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.
Gen Douglas Fraser, the commander of US Southcom, says around 90 aid flights a day are landing.
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