Up to 10,000 US troops will be on the ground or off the coast of Haiti by Monday to help deal with the earthquake aid effort, US defence officials say.
Aid distribution has begun but logistics continue to be extremely difficult, UN officials say.
Tuesday's earthquake has left as many as 45,000-50,000 people dead.
Correspondents say survivors seem increasingly desperate and angry as bottlenecks and infrastructure damage delay relief efforts.
Many are spending another day without food and shelter in the ruined capital.
The US has already sent an aircraft carrier, the USS Carl Vinson, to Haiti and the USS Bataan, carrying a marine expeditionary unit, is on its way.
The chairman of the US Joint Chiefs of Staff, Adm Mike Mullen, said a hospital ship and more helicopters would be sent in the coming days, carrying more troops and marines.
He said the total number of US troops would rise to between 9,000 and 10,000.
"Right now, I mean, literally as we speak, the Vinson (aircraft carrier) and the company from the 82nd Airborne who got there last night are focusing on delivering water from the helicopters offshore to the people of Haiti."
US defence secretary Robert Gates said the relief effort was the "highest priority for US military assets in this hemisphere", and all necessary resources would be made available.
He described infrastructure problems which have led to delays in aid distribution as "facts of life".
"I don't know how ... [the US] government could have responded faster or more comprehensively than it has," he said.
The announcements came after US President Barack Obama pledged full American support in a phone call to his Haitian counterpart Rene Preval.
"Relief Problems"
The UN said a total of about $270m (�165m) in international aid had been pledged so far for the relief effort.
It will launch an emergency appeal for $550m later on Friday, UN spokeswoman Corinne Momal-Vanian said.
UN Secretary General Ban Ki-moon said that distribution of food and medicine was under way, but correspondents say there is little immediate sign of a co-ordinated relief effort on the ground.
The UN's World Food Programme says two million people will need food aid, but it has so far managed to feed just a few thousand.
Meanwhile the head of the charity Medecins Sans Frontieres (Doctors Without Borders) said thousands of people were waiting for surgery in Haiti's hospitals, AFP news agency reported.
Port-au-Prince's small airport is filled to capacity and US air traffic controllers have taken charge to help manage the influx of planes.
The port is too damaged to use and roads are blocked by debris.
The BBC's Andy Gallacher, in Port-au-Prince, says survivors are dying in huge numbers, and clean water, food and medical supplies are desperately needed.
"We hear on the radio that rescue teams are coming from the outside, but nothing is coming," said one man, Jean-Baptiste Lafontin Wilfried.
David Wimhurst, a spokesman for the Brazilian-led UN peacekeeping mission in Haiti, said: "Unfortunately, they're slowly getting more angry and impatient."
The UN headquarters has collapsed and correspondents say there is little official presence in Port-au-Prince despite incidents of looting.
Source: BBC
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