Desperation spawned violence Friday as anguish turned to anger in Haiti, prompting survivors in the shattered, lawless heart of the capital city to resort to frantic looting in a desperate search for the most basic of necessities.

At a collapsed multi-storey shopping complex in downtown Port-au-Prince, looters desperate for food and water braved the rubble to emerge with supplies, only to be swarmed immediately by a mob. Haitian police did little but watch.

While aid workers continued working to extract survivors from the wreckage of Tuesday's earthquake, most Haitians were concerning themselves more with their long-term survival.

The wrecked capital was teeming with people carrying suitcases as they made their way towards the outlying rural areas, which were not as hard hit by the magnitude-7 temblor that brought so much of the city crashing down on Tuesday.

Master Cpl. Christine Briand, an RCMP officer posted with the United Nations, described the security situation as tense and highly volatile, but stable nonetheless.

"The situation is calm for the moment," said Briand, who was working out of a UN logistics base near the airport. "But things can change in an instant here."

The slums on the outskirts of Port-au-Prince, including the notorious Cite Soleil neighbourhood, are becoming violent as residents ransack stores, Canadian police say.

Inside the city, long lines had formed around the few gas stations that had managed to stay open. The price for three gallons worth of gasoline, $15 US a week ago, is now $50 US. Most other stores have shut or emptied their shelves, fearing looters.

Small bands of young men and teenagers with machetes roaming downtown streets helped themselves to whatever they could find in wrecked homes.

"They are scavenging everything. What can you do?" said Michel Legros, 53, as he waited for help to search for seven relatives buried in his collapsed house. A Russian search-and-rescue team said the general insecurity was forcing them to suspend their efforts after nightfall.

"The situation in the city is very difficult and tense," said team chief Salavat Mingaliyev, according to Russia's Interfax news agency.

The main challenge for the international community is resolving the logistical nightmare of co-ordinating aid in a country with next to no telecommunication. Most of the UN's vehicles are being used to escort convoys or transport victims.

Food aid has been further slowed by the extensive damage suffered by the World Food Program's headquarters, whose workers have been forced to recalibrate their distribution efforts.

There is widespread concern that security will degenerate if Haitians are faced with prolonged shortages of food and water.

"It's tough because we first have to provide security for the WFP," Briand said. "But it is important for the population to see that there is food available."

The injured were streaming Friday into already crowded hospitals. At the Hopital la Paix, armed police turned away a surging crowd around the main gate, admitting only the most seriously injured.

A fetid smell filled the corridors where patients were lying on torn mattresses, waiting to be treated. The dead were covered with a simple bedsheet.

"Let me summarize the situation for you, it's catastrophic," said the hospital director, Dr. Marie Yolaine Noel Saint-Fleur. "All the hospital's resources have been used and now we're hoping (the) international (community) will come help us."

Saint-Fleur estimateed her hospital has treated more than 2,000 people since the earthquake. On Thursday alone, they disposed of 250 bodies.

In the days immediately following the catastrophe, bodies littered the streets of the capital, which in 35 C heat created a major health concern. The Haitian government has taken to digging mass graves for the victims. But the health risk remains.

"We're worried about all kinds of epidemics," Saint-Fleur said.

Law enforcement in Haiti, which at the best of times benefits from extensive international mentoring, has been particularly hard hit. Dilapidated police stations around the city collapsed, trapping an untold number of police officers and scattering arrest warrants, witness testimony and other files vital to a now-paralyzed justice system.

UN police mentors say have little idea about how many senior Haitian national police officers survived the quake.

Hard-pressed government workers, meanwhile, buried thousands of bodies in mass graves. The Red Cross has estimated 45,000 to 50,000 people were killed in the quake, although Haitian authorities now estimate the death toll at more than 140,000.

Roughly half the buildings in the capital and other hard-hit areas have been either damaged or destroyed, U.N. Secretary-General Ban Ki-moon said in New York.

Ordinary Haitians sensed the potential for an explosion of lawlessness. "We're worried that people will get a little uneasy," said attendant Jean Reynol, 37, explaining his gas station was ready to close immediately if violence breaks out.

"People who have not been eating or drinking for almost 50 hours and are already in a very poor situation," U.N. humanitarian spokeswoman Elisabeth Byrs said in Geneva. "If they see a truck with something, or if they see a supermarket which has collapsed, they just rush to get something to eat."

The logistical obstacles were many. "There are a lack of trucks, lack of fuel, blocked roads and so on," U.N. humanitarian chief John Holmes said in New York.

Water trucks have set up distribution points in various corners of the city.

With uncertainty over access to basic resources, Canadians were continuing to line up outside the embassy, hoping to board a military plane back to Canada.

Around 75 people were seeking shelter in the embassy compound, jammed into a tennis court and small garden. Embassy workers are bringing food from their homes to help feed the evacuees.

More than 250 Canadians had been evacuated by Friday afternoon.

Canada's Department of Foreign Affairs said there are some 1,415 Canadians officially listed as missing in Haiti, a number that may be inflated because of communications problems. The government estimates there are some 6,000 Canadians living in the country.

Among those still missing is the senior Canadian police officer in the country, the RCMP's Doug Coates.

Up to 10,000 U.S. troops were expected in Haiti or off its shores by Monday to distribute aid and prevent potential rioting among desperate earthquake survivors.

Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, said the total U.S. presence in and around the beleaguered country could rise beyond 10,000 as U.S. military officers determine how much assistance may be needed in the days ahead.

Hundreds of U.S. troops touched down in Port-au-Prince overnight and were handing out food and water to stricken survivors, including more than 100 paratroopers of the U.S. 82nd Airborne Division, boosting the U.S. military presence to several hundred on the ground here. Others have arrived off Port-au-Prince harbour on the aircraft carrier USS Carl Vinson.

As temperatures rose, a stench of death lingered over Port-au-Prince, where countless bodies remained unclaimed in the streets. Hundreds of corpses were stacked outside the city morgue, and limbs of the dead protruded from the rubble of crushed schools and homes.

Experts say people trapped by Tuesday's quake would begin to succumb if they go without water for three or four days.

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

In his third such update from the White House in as many days, President Barack Obama promised an expansive U.S. effort to help Haiti survive its disaster, not just in the push to save lives but as part of a longer-term effort to help rebuild the country.

Obama, said he spoke for 30 minutes with Haitian President Rene Preval and pledged full U.S. support for both the immediate recovery effort and the long-term reconstruction.

"The scale of the devastation is extraordinary, as I think all of us are seeing on television," Obama said. "The losses are heartbreaking."

Obama was scheduled to meet Saturday with the two presidents who preceded him in the Oval Office, George W. Bush and Bill Clinton, in an effort to get the American people more broadly involved in the recovery effort.

In a joint news conference at the Pentagon with Mullen, Defence Secretary Robert Gates said the primary goal is to distribute aid as quickly as possible "so that people don't, in their desperation, turn to violence.

With files from the Associated Press