Violence and looting are sweeping through the Haitian capital.
Hundreds of thousands of hungry Haitians are still waiting for help, many of them in makeshift camps on streets strewn with debris and decomposing bodies.
New estimates put the quake death toll at as many as 200,000; but six days into the crisis, aid distribution was still random, chaotic and minimal, though world leaders have pledged massive assistance to rebuild.
International medical teams have taken over damaged hospitals and clinics where wounded and sick people have lain untreated for days.
In one incident a starving crowd clamoured to snatch food and supplies from shops. In another, residents caught two suspected looters, tied them together, beat them and dragged them through the streets.
Gangs of men, their faces covered with bandanas to mask both their identity and the smell of decaying bodies, brandished machetes and sharpened planks of wood as they ran from shop to shop stealing shoes, rolls of carpet and cooking pots.
Two aid workers from the Dominican Republic were shot and seriously wounded as they handed out food. Carlos Gatas and Milton Matos struggled back to their embassy with gunshot wounds after the attack.
There were also reports of hungry people fighting with machetes over small packages of food that were dropped into a stadium by a helicopter.
Despite being outnumbered, police officers tried to disperse the violence, but elsewhere there were reports that officers allowed vigilantes to deal with looters.
It is hoped the US troops will be able to quell the unrest, which has forced some rescue workers to pull out.
Canada has also promised to send 1,000 troops to support the relief effort, in addition to the 500 already there.
Only trickles of aid were getting through as much of the relief effort was hampered by the logistical problem of transporting supplies from the small and damaged airport into the city.
Medecins Sans Frontieres said that a cargo plane carrying an inflatable field hospital with the capacity to hold more than 100 patients was denied permission to land and had to be re-routed through the Dominican Republic, causing a 24-hour delay.
The US army is working to open a critical port in Haiti to help the flow of aid and helicopters have been drafted in to distribute supplies.
So far, they have delivered more than 70,000 bottles of water and 130,000 food rations.
Lieutenant General PK Keen, deputy commander of US Southern Command said: "We're going to be able to increase that every day.
"But, clearly, this is a disaster of epic proportions, and we've got a lot of work ahead of us."
Vast queues formed at distribution points where the UN World Food Programme handed out high-energy food, which had been delivered under armed guard.
Florence Louis, (29), a seven months pregnant mother of two, was one of thousands of Haitians who gathered at a gate at the Cite Soleil slum.
Clutching four packs of biscuits, she said: "It is enough because I didn't have anything at all."
On another street, six young men ripped water pipes off walls to suck out the few drops inside. "This is very, very bad, but I am too thirsty," said Pierre Louis Delmar.
Adding to the security fears, heavily armed gang members who once ran Haiti's worst slum like warlords have returned to the Cite Soleil shantytown since breaking out from prison after the 7.0 magnitude earthquake struck on Tuesday.
The UN said increasing numbers of Haitians were trying to cross the border into the Dominican Republic, on the eastern side of Hispaniola island, and reported a surge of quake survivors fleeing to northern cities.
UN Secretary-General Ban Ki-Moon visited Haiti yesterday. He said: "This is one of the worst humanitarian crises in decades.
"The damage, destruction, loss of life is just overwhelming,"
Mr Ban's first stop was at the collapsed UN mission headquarters in the capital, Port-au-Prince, where rescuers worked feverishly to extract a Danish employee from the rubble. Staffers accompanying Mr Ban wiped away tears as they viewed the destruction and mourned their dead and missing colleagues.
About 15 minutes after Mr Ban left, emergency workers successfully pulled the man from the building. He was talking, was given some water and taken to a hospital, UN staffers travelling with Mr Ban said.
The UN said it was feeding 40,000 people a day and hoped to increase that to one million within two weeks.
How you can help
The Disasters Emergency Committee – the umbrella organisation for 13 aid agencies including the British Red Cross and Oxfam – has launched an emergency appeal to help the victims of the Haiti earthquake.
If you would like to make a donation, please call 0370 60 60 900 or visit www.dec.org.uk