Police fire gun shots to disperse looters in Port-au-Prince
More than 2,000 US Marines are set to join 1,000 US troops on the ground in Haiti, as aid efforts gather pace a week after a devastating earthquake.
Their arrival comes amid violence and looting, and as UN and US forces pushed back a crowd at Port-au-Prince's airport gate by firing baton rounds.
UN head Ban Ki-moon said he would recommend a 3,500 boost to police and troop numbers to the Security Council.
People have been continuing to flee the capital, with many seeking US visas.
The BBC's Mark Doyle in Port-au-Prince, the capital, says perhaps 5,000 people have lined up outside the US embassy, desperate to join relatives among the large Haitian-American community in the US.
But UN humanitarian chief John Holmes played down worries over security, saying that despite incidents of violence, the overall situation was calm.
AT THE SCENE
Matt Frei, BBC News, Port-au-Prince
Looting is now the only industry here and this is the new rush hour of Port-au-Prince.
Anything will do as a weapon: a hacksaw, a stick, and of course all the machetes and guns that you cannot see.
Patience is running out and all the ingredients for unrest now exit: a whole city of destitute hoping for help, and at the same time you have a substantial criminal element and a history of violence. None of this bodes well for Haiti.
If the anarchy spreads, the US troops may soon find themselves patrolling the streets in what will look like a full-scale military operation.
And the leading US general in Haiti, Lt Gen Ken Keen, said there was currently less violence in Port-au-Prince - already a troubled city - than there had been before the earthquake.
Earlier, Gen Keen said up to 200,000 people might have died in the disaster, which he said was of "epic proportions".
Aid effort 'improving'
More than 2,200 US Marines arrived off the coast of Haiti on Monday aboard the amphibious ship USS Bataan, US media reported.
They are equipped with heavy-lifting and earth-moving equipment, a dozen helicopters and medical support facilities.
Delivering aid to the centre of the capital is getting much more difficult, as anger fuelled by hunger reaches boiling point, and military escorts are needed for lorries carrying supplies, the BBC's David Loyn in Port-au-Prince says.
US Navy helicopters have been dropping packages of ready-to-eat meals and water from the air, but they can only feed a few people at a time, our correspondent adds.
The World Food Programme (WFP) has said it is now feeding some 100,000 people, stressing that security is a major challenge.
Bill Clinton: 'There's no question it's not enough, not quick enough'
Former US President Bill Clinton, who is a UN Special Envoy to Haiti, visited the General Hospital in Port-au-Prince on Monday.
He said he had been told before his trip that it needed medicine and power generators.
"We need very specific things that there is a shortage of," he said. "That is what I tried to do today. I called this hospital... and we said tell us very specifically everything you need and that's what we brought down."
Lt Cdr Walter Matthews of the US Navy told the BBC he understood the frustration among Haitians but that the aid effort was improving.
Luca Pupulin, of the French charity Acted, told the BBC he had seen many people heading north away from the capital.
He said: "I think on the whole the community is trying to do its best, but they are getting very frustrated.
"A couple of days ago we went through the main market street - people were getting aggressive with each other and pillaging."
"The security situation is getting worse by the hour. I think the criminal gangs, totalling some 3,000, are going to exploit the current humanitarian crisis to the maximum degree," Stuart Page, the head of a security company operating in Port-au-Prince, told the BBC.
The BBC's Matt Frei, in Port-au-Prince, says looting is now the main industry in the city and is being run by rival gangs.
Especially prized is toothpaste, which people smear under their noses to fend off the stench of decaying corpses, the Associated Press news agency says.
Port-au-Prince's port is badly damaged, and many roads are still blocked by corpses and debris, hampering the delivery of fuel and other supplies.
More than 70 people have been pulled from the wreckage in the last few days
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