The BBC's Matt Frei says there has been gunfire and looting in some areas
US Secretary of State Hillary Clinton has become the first senior Western official to arrive in Haiti after the earthquake that flattened the capital.
Mrs Clinton told the Haitian people that the US would be "here today, tomorrow and for the time ahead".
Tens of thousands of people were killed and survivors have grown desperate as they wait for aid to arrive.
The UN has meanwhile confirmed the head of its mission in Haiti has been found dead in the rubble of its headquarters.
Earlier, Chinese rescuers told the Xinhua news agency that the body of Hedi Annabi, a Tunisian, had been recovered from the building in Port-au-Prince at 1422 local time (2022 GMT) on Saturday.
You have been severely tested, but I believe that Haiti can come back even better and stronger in the future
In a statement issued shortly afterwards, UN Secretary General Ban Ki-moon also confirmed the death of Mr Annabi's deputy, Luiz Carlos da Costa of Brazil, and the acting police commissioner, Doug Coates of Canada.
"Their dearest wish, I'm sure, would be that we carry forward the noble work that they and their colleagues performed so well," said Mr Ban.
Criticism 'not fair'
Mrs Clinton, who is being accompanied by US Agency for International Development (USAID) administrator Rajiv Shah, arrived on a US Coast Guard C-130 carrying aid and is to leave on another with evacuees.
Clinton seeks to reassures Haiti
After discussing the relief effort with the President Rene Preval and Prime Minister Jean-Max Bellerive at the airport, she told a news conference that the United States was "a friend, a partner and a supporter" of Haiti.
"We are here at the invitation of your government to help you," she told Haitians. "We will be here today, tomorrow and for the time ahead."
"Speaking personally, I know of the great resilience and strength of the Haitian people. You have been severely tested, but I believe that Haiti can come back even better and stronger in the future."
Earlier, Mrs Clinton dismissed criticism that aid was not leaving the airport and being delivered to the people who needed it, saying 14 distribution centres were being established around the capital to deliver food and water to the two million people that the US and UN estimates need emergency aid.
AT THE SCENE
Mark Doyle, BBC News, Haiti
The scene an hour's drive west of Port-au-Prince is apocalyptic. Almost every single building on the road I'm driving on now has been flattened. Reinforced concrete roofs have fallen to the floors below, or have jammed into the ground at crazy angles.
The destruction here is even more dramatic than the dreadful conditions in the capital. People have fled to the surrounding sugarcane fields or into mangrove swamps - anything to get away from the nightmare of the falling buildings.
I've seen a long line of people queuing up at a single working water tap. Tens of thousands are living in the open in church compounds, school playgrounds and marketplaces.
The population here are in profound shock. Many are wearing handkerchiefs over their mouths to keep out the dust and ward of the smell of dead bodies.
The US military is also beginning to use a container port in Cap Haitian, in northern Haiti, which will greatly increase the volume of aid flowing into the country. Port-au-Prince's dock remains inoperable.
Reports coming out of Port-au-Prince had earlier indicated that a crowd of about 1,000 people was involved in a violent fight over goods in one of the city's central commercial streets.
A photographer for the Reuters news agency said that a group of men armed with knives, ice-picks, and hammers fought one another over clothing, toys and any other items they could find in destroyed houses and shops.
"It's anarchy there now, total chaos, the police have gone away," Chris Barria said. "They are fighting, hitting each other, throwing stones at each other."
But the UN Under-Secretary General for Humanitarian Affairs, John Holmes, played down the incidents of violence. He told the BBC that the security situation in Haiti was calm, minor incidents apart.
Disaster 'like no other'
Estimates of how many people died following the 7.0 magnitude earthquake on Tuesday have varied.
The Pan American Health Organization put the death toll at 50,000-100,000, while Haitian Prime Minister Jean-Max Bellerive said 100,000 "would seem a minimum".
He said more than 25,000 bodies had been collected and buried. Rescue workers have been racing against time to find survivors still trapped under the rubble.
It's worse than the Indonesian earthquake where at least we could get the support of some local authorities
Elisabeth Byrs UN Office for the Co-ordination of Humanitarian Affairs
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