Hundreds of U.S. troops are now on the ground in Haiti, working to stabilize the nation's shattered capital and distribute aid to millions of people who have been scavenging for food and shelter.

Recent developments:

• Up to 10,000 U.S. troops in Haiti by Monday
• Delivery of aid slowed by obstacles
• UN estimates a third of all urban buildings ruined
• Thousands of victims buried in mass graves

Nearly four days after the 7.0-magnitude quake struck the Haitian capital, bodies are littered on city streets, and the UN believes that at least 300,000 people are homeless and starving.

While aid began to trickle into the nation, the hope of finding survivors among the collapsed buildings of Port-au-Prince was beginning to fade.

"I would think the rescue teams have another couple of days before this becomes a search for bodies," said CTV's Paul Workman on Friday night, reporting from Port-au-Prince.

"The heat is punishing and anyone still buried alive will need water badly."

Elsewhere, Haitians in Port-au-Prince, some armed with machetes, searched collapsed buildings for anything edible, The Associated Press reported.

"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.

United Nations peacekeepers say the tension is rising among survivors and they warned aid convoys to add security.

"Unfortunately, they're slowly getting more angry and impatient," said David Wimhurst, 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."

Quake survivors are tired, hungry, stressed and acutely aware of the threat of lawlessness in Port-au-Prince.

"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.

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.

Further complicating the situation is the estimated 4,000 criminals freely roaming the streets after the city's main prison collapsed in Tuesday's disaster, said International Red Cross spokesperson Marcal Izard.

The UN says it has been able to keep things under control so far.

"It's tense but they can cope," said Elisabeth Byrs, a UN humanitarian spokesperson.

Bodies collected, buried

Survivors have been rooting through rubble of flattened buildings for days now, looking for survivors or the remains of loved ones and friends.

Hundreds of bodies were stacked outside the city morgue on Friday. Workers were using bulldozers to transport loads of bodies away from the streets.

The exact number of dead is still unknown, though the international Red Cross estimates that between 45,000 and 50,000 people were killed in the disaster.

CTV's Daniele Hamamdjian, reporting from the capital, spoke of destruction and chaos.

"It's overwhelming to see the number of bodies on the side of the road, it's heartbreaking to see children's bodies covered in sheets," she said.

Government crews had buried more than 9,000 corpses in mass graves after collecting them from the streets and morgues over a 20-hour period.

Others, like Jawil Bay and his friends, are burying the dead to help the living.

"I am working for my country," Bay said on Friday.

Elsewhere, there have been incredible tales of survival. One American man, trapped in the rubble for 20 hours, was freed by rescue workers after a formidable effort.

The man lost a leg from the knee-down, in addition to his other foot, but he was alive, Hamamdjian reported.

Donations pour in

Foreign governments and agencies have said they will give $400 million worth of aid, including $100 million from the United States.

And more than 20 governments have rushed food, water, tents, blankets, water-purification gear, heavy equipment and other materials to Haiti to provide relief.

The UN has also asked for an additional emergency flash appeal of $560 million in aid, in addition to the bilateral funds already pledged.

President Barack Obama reiterated Friday that the U.S. has a responsibility to help its Caribbean neighbour. He has enlisted former presidents George W. Bush and Bill Clinton to assist with private fundraising efforts for Haiti, he said.

U.S. Secretary of State Hillary Rodham Clinton said she would travel to Haiti on Saturday to inspect the damage and to show "our long term, unwavering support, solidarity and sympathies."

But such well-intentioned efforts have come up against the limitations of a quake-ravaged Port-au-Prince -- a capital city with a damaged seaport and an airport that doesn't have enough fuel or space for all incoming flights to land.

Aid workers who have made it into Haiti have been blocked by debris strewn across inadequate roads and by survivors gathered in the streets.

UN Secretary-General Ban Ki-moon said food distribution had begun in Port-au-Prince and that he would travel there to assess the situation.

"A high proportion of the three million people in the capital area are without access to food, water, shelter and electricity," he said, adding that the massive effort "is inevitably slower and more difficult than any of us would wish."

He also acknowledged that tensions were rising, along with concern over the possibility of violence.

He said that 3,000 U.N. troops and police, who are walking the streets to keep the peace, "are taking all possible precautionary measures."

He added that about half of all the buildings in the capital have been wrecked and that about 8,000 people are being fed high-energy biscuits by UN workers.

U.S. Adm. Mike Mullen, the highest ranking U.S. military officer, said Friday that he expects 9,000 to 10,000 American troops will be in Haiti by Monday. They will help distribute aid and prevent potential rioting.

With files from The Associated Press