The United Nations says the Haiti earthquake is the worst disaster it's ever confronted.
The UN Office for the Co-ordination of Humanitarian Affairs in Geneva says the country has been "decapitated".
It says the situation is worse than the devastation wrought by the 2004 tsunami in the Aceh province of Indonesia.
Haiti's capital Port-au-Prince was extensively damaged by a magnitude 7.0 earthquake on Tuesday.
Aftershocks are still being felt every few hours. The US Geological Survey recorded measured one at magnitude 4.5, on Saturday morning.
Gangs are preying on residents and looting. Officials say 4000 prisoners are unaccounted for after the main prison was destroyed.
There are reports of people fighting each other for food. Many people are trying to get out of Port-au-Prince.
The BBC reports there is little police presence in the capital, although some Brazilian UN peacekeepers are patrolling the streets.
Relief has been arriving, but little has moved beyond the jammed airport.
Damage to the port, roads and other infrastructure has prevented distribution of food, water and medical supplies.
Interior Minister Paul Antoine Bien-Aime said 50,000 bodies had been collected, but the total number of dead could be "between 100,000 - 200,000".
Trucks piled with corpses have been carrying bodies to mass graves.
The UN said on Friday that about 300,000 people had been made homeless.
Foreign assistance
US Secretary of State Hillary Clinton arrived in Haiti on Saturday to express "long-term, unwavering support, solidarity and sympathies". She was to meet Haitian President Rene Preval.
UN Secretary General Ban Ki-moon is due to visit on Sunday.
The United States has already sent an aircraft carrier, the USS Carl Vinson, to Haiti and the USS Bataan, carrying marines, is on its way.
A hospital ship, the USS Comfort, and more helicopters are also being sent.
The total number of US troops there will be between 9000 - 10,000.
The Comfort previously delivered medical care to Haiti after extensive flooding and mudslides in 2008.
The United States has taken temporary control of the airport in Port-au-Prince to help distribute aid more quickly.
The UN has launched an appeal for $US562 million intended to help three million people for six months.
A total of about $US360 million has been pledged so far for the relief effort, but only part of this sum will be included in the emergency appeal.
President Barack Obama has met two of his predecessors in the White House, George W Bush and Bill Clinton. Theywill lead US fundraising efforts through the Bush-Clinton Haiti Fund.
Other damage
Meanwhile, the ABC reports details are emerging about the extent of the damage beyond Port-au-Prince.
The UN says up to 90% of the buildings have been damaged in Leogane, a town about 19km to the west.
At Carrefour, a town with 334,000 inhabitants, 40% - 50% of buildings in the worst-affected areas had been destroyed.
In Gressier, which has 25,000 inhabitants, around 40% - 50% of the buildings - including the police station - had been destroyed.
