Concerned aid workers in Haiti today called for more security amid fears of attacks by increasingly desperate earthquake survivors.

Tens of thousands are feared dead from Tuesday's massive quake in the Caribbean island nation. The Pan American Health Organisation estimated the death toll could be 50,000 to 100,000.

US president Barack Obama said the earthquake had inflicted "heartbreaking" losses and pledged America would do what it takes to save lives and get the country back on its feet.

"The scale of the devastation is extraordinary ... and the losses are heartbreaking," Mr Obama said at the White House.

UN secretary general Ban Ki-moon said he planned to go to Haiti "very soon".

UN peacekeepers patrolling the capital Port-au-Prince said there was a growing feeling among people that aid was not being distributed quickly enough.

And the Brazilian military warned aid convoys to guard against looting.

“Unfortunately, they’re slowly getting more angry and impatient,” said a spokesman for the Brazilian-commanded UN peacekeeping mission. “I fear, we’re all aware that the situation is getting more tense as the poorest people who need so much are waiting for deliveries. I think tempers might be frayed.”

The UN World Food Programme said its warehouses in the capital had been looted and it did not know how much of its pre-quake stockpile of 15,000 tons of food aid remained.

A spokeswoman added that regular food stores in the city also had been emptied by looters. She said the WFP was preparing shipments of enough ready-to-eat meals to feed two million Haitians for a month.

The International Red Cross estimated 45,000 to 50,000 people were killed in the quake on Tuesday. About three million people, one third of Haiti's population, were hurt or left homeless by the major 7.0 magnitude quake that hit its impoverished capital on Tuesday.

At least 10 per cent of housing in the capital was destroyed, but in some areas half of all buildings were destroyed or badly damaged, according to a preliminary assessment by UN disaster experts.

Hundreds of bodies are stacked outside the city morgue, and limbs of the dead protruded from the rubble of crushed schools and homes. A few workers were able to free people who had been trapped for days, but others attended to the grim task of using bulldozers to transport loads of bodies.

For the long-suffering people of Haiti, the Western Hemisphere’s poorest nation, shock was giving way to despair. Desperate Haitians blocked streets with corpses in one part of Port-au-Prince to demand quicker relief efforts following the quake.

“We need food. The people are suffering. My neighbours and friends are suffering,” said one man. “We don’t have money. We don’t have nothing to eat. We need pure water.”

Some 20 governments from Europe, Asia and the Americas, the UN and private aid groups were sending planeloads of high-energy biscuits and other food, tons of water, tents, blankets, water-purification gear, heavy equipment for removing debris, helicopters and other transport. Hundreds of search-and-rescue, medical and other specialists also headed to Haiti.

Governments and government agencies have pledged about $400 million of aid. A UN spokeswoman said UN aid agencies will launch an emergency appeal for approximately $550 million in New York today.

US president Barack Obama pledged an initial $100 million (€69 million) for Haiti quake relief yesterday and enlisted former US presidents Bill Clinton and George W Bush to help raise more, vowing to the Haitian people: "You will not be forsaken."

The US military aimed to have about 1,000 troops on the ground in Haiti today, and thousands more in ships off shore. The total will reach 9,000-10,000 troops by Monday.

The Irish government said this evening it would ship 85 tonnes of emergency supplies.

The Cuban government has agreed to let the US military use restricted Cuban air space for medical evacuation flights carrying Haitian earthquake victims, sharply reducing the flight time to Miami.

White House spokesman Tommy Vietor said a deal had been reached allowing evacuation flights from the US Naval Station in Guantanamo Bay, Cuba, to pass over the island on the way to Florida. The deal would shorten the flight time by 90 minutes on trips that normally are routed around Cuba.

But into the third day following the quake, the global helping hand was slowed by a damaged sea port and an airport that turned away civilian aid planes for eight hours yesterday because of a lack of space and fuel.

Aid workers have been blocked by debris on inadequate roads and by survivors gathered in the open out of fear of aftershocks and re-entering unstable buildings.

Many hospitals were too battered to use, and doctors struggled to treat crushed limbs, head wounds and broken bones at makeshift facilities where medical supplies were scarce. Makeshift tents were strung everywhere.

Across the sprawling, hilly city, people milled about in open areas, hoping for help, sometimes setting up camps amid piles of salvaged goods, including food scavenged from the rubble.

Police have all but vanished from the streets, and although some Brazilian UN peacekeepers were patrolling, there were reports of sporadic scavenging and some looting.

Some looting broke out in downtown Port-au-Prince today and shots were fired, a witness said. "I heard the shots and got out of there. The police told us it was too dangerous to stay. People were looting and a body was being burned," said a foreign photographer, who asked not to be identified.

At one destroyed supermarket scores of people swarmed over the rubble to try to reach the food underneath. Just outside Cite Soleil slum, desperate people crowded around a burst water pipe jostling to drink from the pipe or fill up buckets.

Some survivors, angry over the delay in getting aid, built roadblocks with corpses on Thursday in one part of the city.

Small groups could be seen burying dead by roadsides. Other dust-covered bodies were being dragged down streets, toward hospitals where relatives hoped to leave them. Countless dead remained unburied, some in piles.

Engineers from the UN mission have begun clearing some main roads, and law-and-order duties have fallen completely to the mission’s 3,000 international troops and police. About 5,500 US soldiers and Marines were expected to be in Haiti by Monday. Their efforts will include providing security.

The UN said at least 36 members of its 9,000-strong peacekeeping mission had been killed and scores remained missing. Brazil said 14 of its soldiers were among the dead.

Fourteen people were pulled alive yesterday from the landmark Montana Hotel, which was largely flattened.

Agencies