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As 2 million people struggle to maintain a semblance of life in earthquake-shattered Port-au-Prince, the Haitian capital has reached a tipping point: in less than 24 hours there is likely to be violence in the streets if food and water remain scarce and distribution choked by unpassable roads.

The calm and dignity of most residents is near breaking, as pockets of looting flared across the city Friday. Small bands of young men and teenagers with machetes roamed downtown streets and helped themselves to whatever they could find in wrecked homes.

Hundreds of U.S. troops touched down in the city overnight and were soon handing out food and water to stricken survivors, but relief groups struggled to deliver aid Friday and fears spread of unrest on Haiti's fourth day of desperation.

"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.

SLIDESHOW: Devastation in Haiti

Even as the U.S. military began distributing aid, Haiti's U.S. ambassador, Raymond Joseph, made a desperate plea Friday for body bags to help manage the sea of corpses that is swelling around the morgue in Port-au-Prince and lining the city's streets.

"We want body bags, because the way they're disposing of some of the corpses is very undignified," Joseph said during brief remarks at the Greater Washington Hatian Relief Center.

"We want ... a lot of body bags. If you know how to get them, if you know a company that makes them and wants to donate or even sell them."

Hard-pressed government workers, meanwhile, were burying thousands of bodies in mass graves. The Red Cross estimates 45,000 to 50,000 people were killed in Tuesday's cataclysmic earthquake.

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More and more, the focus fell on the daunting challenge of getting aid to survivors. United Nations peacekeepers patrolling the capital said people's anger was rising that aid hasn't been distributed quickly, and warned aid convoys to add security to guard against looting.

Angry Haitians have reportedly been using corpses to set up roadblocks in Port-au-Prince to protest those delays. Photographer Shaul Schwarz said he saw at least two downtown roadblocks formed with bodies of earthquake victims and rocks.

"They are starting to block the roads with bodies," he told Reuters. "It's getting ugly out there, people are fed up with getting no help."

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 quake's destruction of Port-au-Prince's main prison complicated the security situation. International Red Cross spokesman Marcal Izard said some 4,000 prisoners had escaped and were freely roaming the streets.

"They obviously took advantage of this disaster," Izard said.

But Byrs said peacekeepers were maintaining security despite the challenges. "It's tense but they can cope," she said.

The U.N. World Food Program said post-quake looting of its food supplies long stored in Port-au-Prince appears to have been limited, contrary to an earlier report Friday. It said it would start handing out 6,000 tons of food aid recovered from a damaged warehouse in the city's Cite Soleil slum.

A spokeswoman for the Rome-based agency, Emilia Casella, said the WFP was preparing shipments of enough ready-to-eat meals to feed 2 million Haitians for a month. She noted that regular food stores in the city had been emptied by looters.

At the airport, some 160 frantic and exhausted U.S. citizens, along with others stranded there for days, were begging for evacuation. "We've had people crying, people passing out," said Muriel Sinai, 38, a nurse from Orlando, Fla. American soldiers were sorting out the Americans, but it wasn't clear whether and when they might be flown out.

The U.S. ambassador to Haiti says the government has no way to check on the 40,000 or more Americans currently estimated to be in the country.

About 450 Americans have been evacuated, said Ambassador Kenneth Merten, and the State Department has been in contact with about 1,000 in all. Six U.S. citizens were confirmed to have died in the earthquake, including a State Department employee.

More than 100 paratroopers of the U.S. 82nd Airborne Division arrived at the Port au Prince airport overnight, boosting the U.S. military presence to several hundred on the ground here, and others have arrived off Port-au-Prince harbor on the aircraft carrier USS Carl Vinson.

The command said other paratroopers and Marines would raise the U.S. presence to 8,000 troops in the coming days. Their efforts will include providing security, White House spokesman Robert Gibbs said.

The Obama administration also said Friday it will allow Haitians who are in the United States illegally to remain because of this week's catastrophic earthquake.

Homeland Security Secretary Janet Napolitano granted the temporary protected status on Friday, two days after she temporarily halted deportations of Haitians, even those already in detention. The protection is available only to Haitians already in the country as of last Tuesday, when the quake struck their home island. They will be allowed to stay and work for 18 months.

Hundreds of bodies were 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 under the rubble for days, including a New Jersey woman, Sarla Chand, 65, of Teaneck, freed by French firefighters Thursday from the collapsed Montana Hotel. But others attended to the grim task of using bulldozers to transport loads of bodies.

Driving a yellow backhoe through downtown Friday morning, Norde Pierre Rico said his government crew had cleared one house and found four people alive. But "there's no plan, no dispatch plan," he said, another sign of a lack of coordination and leadership in the rescue and aid efforts in these early days of the crisis.

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

Haitian President Rene Preval told The Miami Herald that over a 20-hour period, government crews had removed 7,000 corpses from the streets and morgues and buried them in mass graves.

For the long-suffering people of Haiti, the Western Hemisphere's poorest nation, shock was giving way to despair.

"We need food. The people are suffering. My neighbors and friends are suffering," said Sylvain Angerlotte, 22. "We don't have money. We don't have nothing to eat. We need pure water."

From Europe, Asia and the Americas, more than 20 governments, the U.N. 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.

But the global helping hand was slowed by a damaged seaport and an airport that turned away civilian aid planes for eight hours Thursday because of a lack of space and fuel.

At Toussaint L'Ouverture International Airport, a stream of U.S. military cargo planes was landing Friday, but they had to circle for an hour before getting clearance to land because the quake destroyed the control tower and radar control, and the U.S. military was using emergency procedures.

Puerto Rico's search-and-rescue team is still on the ground a short flight away from Haiti and has been waiting for nearly three days to get to their neighbors in need. In San Juan, more than 1 million bottles of water and massive amounts of canned goods and dried milk sit unused, waiting to be deployed.

International support has clogged the one small entry into Haiti, an airfield with a single working airstrip.

Aid workers on the ground have been blocked by debris on inadequate roads and by survivors gathered in the open out of fear of aftershocks from the 7.0-magnitude quake and re-entering unstable buildings.

"The physical destruction is so great that physically getting from point A to B with the supplies is not an easy task," Casella, the WFP spokeswoman in Geneva, said at a news conference.

Aid worker Fevil Dubien said some people were almost fighting over the water he distributed from a truck in a northern Port-au-Prince neighborhood.

Elsewhere, about 50 Haitians yearning for food and water rushed toward two employees wearing "Food For The Poor" T-shirts as they entered the international agency's damaged building.

"We heard a commotion at the door, knocking at it, trying to get in," said project manager Liony Batista. "'What's going on? Are you giving us some food?' We said, 'Uh-oh.' You never know when people are going over the edge."

Batista said he and others tried to calm the crowd, which eventually dispersed after being told food hadn't yet arrived.

"We're not trying to run away from what we do," Batista said, adding that coordinating aid has been a challenge. "People looked desperate, people looked hungry, people looked lost."

Engineers from the U.N. mission have begun clearing some main roads, and law-and-order duties have fallen completely to its 3,000 international troops and police.

David Wimhurst, spokesman for the U.N. peacekeeping mission, said Haitian police "are not visible at all," no doubt because many had to deal with lost homes and family members. The first U.S. military units to arrive took on a coordinating role at the airport.

Batista, the Food For The Poor project manager, went back to the Dominican Republic late Thursday and awaited the arrival of 100 shipping containers loaded with rice, canned goods and building supplies.

"I don't think that a word has been invented for what is happening in Haiti," he said. "It is total disaster."

Fox News' Jonathan Hunt, Adam Housley and the Associated Press contributed to this report.