The US says it is doing everything in its power to speed aid to Haiti
The US is sending another 4,000 sailors and marines to Haiti for the earthquake relief effort, diverting them from deployments in the Gulf and Africa.
The 24th Marine Expeditionary Unit and USS Nassau Amphibious Ready Group would "significantly" increase the ability to quickly provide aid, the navy said.
The move will increase the number of US troops involved to about 16,000.
Earlier, a strong aftershock rocked the capital, Port-au-Prince, but did not seem to cause further casualties there.
Away from the city, survivors in some towns are still waiting for help after the original 7.0-magnitude earthquake struck eight days ago.
In the village of Leogane, near the quake's epicentre, doctors from Spain and Haiti's neighbour, the Dominican Republic, have set up a treatment centre.
Surgeons from the US and Cuba are also there, working together.
One of the US doctors said: "We've been so pleased with our interaction with the Cubans."
A member of the Cuban team said: "We're professional doctors and can work with doctors from any country in the world."
The BBC's Mark Doyle, reporting from Leogane, said the village had been left looking worse than most war zones - as if it had been carpet-bombed, then pounded with a relentless artillery barrage.
Haitian officials say the number who have died as a result of the quake is likely to be between 100,000 and 200,000, and that 75,000 bodies had already been buried in mass graves.
But UN officials say it is likely the actual number of people who have died will never be established.
An estimated 1.5 million Haitians are homeless.
People are still being found alive in the rubble, including a five-year-old boy reportedly pulled from the ruins of his home by his uncle. Earlier a 10-year-old girl and her eight-year-old brother were found.
More than 120 people altogether have been rescued by international teams.
Rescuers applaud as the children are pulled alive from the ruins
'Array of assets'
On Wednesday afternoon, the US Second Fleet announced that the three-ship USS Nassau Amphibious Ready Group (ARG), which left port in Virginia on Monday for its regular deployment, had been told to go to Haiti instead.
We were forced to buy a saw in the market to continue amputations
The group will pick up the 24th Marine Expeditionary Unit (MEU) from North Carolina en route, making up a force of 2,000 sailors and 2,000 marines.
Adm Mike Mullen, the chairman of the US Joint Chiefs of Staff, made the decision to divert the troops "based on continuing urgent needs in the Haiti relief effort".
"The ARG/MEU will provide an array of helicopter and amphibious landing craft assets, significantly increasing the ability to quickly provide relief supplies where they are most needed," the Second Fleet said in statement.
"In addition, the marines assigned to 24 MEU will be able to provide an additional force capable of providing a secure environment for the ongoing relief efforts ashore in Haiti."
The first group of 2,000 marines on board vessels off Haiti's coast went ashore on Tuesday to assist the relief effort. They have since landed heavy equipment, such as bulldozers, diggers and trucks, and other supplies near the epicentre of last week's earthquake.
AT THE SCENE
Adam Mynott, BBC News, Port-au-Prince
This, we are told by the international medics who have come here, is one of the better-organised hospitals in the city.
We are standing by the entrance, which has in effect been turned into a ward.
It is a quite extraordinary sight. There are people with injuries lying everywhere, some on beds, some on mattresses, some just lying on bits of cloth stretched out on the stone floor.
The doctor says that one of the big risks now is that of secondary infection.
Outside the hospital, what once was the car park has been turned into an outpatients' department and there must be more than 100 patients here.
Many of them have been treated and could go home but, of course, they have no homes to go to.
The decision to redeploy assets by the US military came after it defended its handling of the rescue operation. Survivors and aid groups have complained of long delays in getting vital supplies of food, water and medicine.
Medicins Sans Frontieres (Doctors Without Borders) said its cargo plane with 12 tonnes of medical supplies had been turned away from the congested Port-au-Prince airport three times since Sunday. It said five patients died from lack of the supplies it carried.
"We were forced to buy a saw in the market to continue amputations," the group's Loris de Filippi told the Reuters news agency in Cite Soleil.
Gen Douglas Fraser, head of US Southern Command, insisted US forces were "doing everything in our power to speed aid to Haiti as fast as humanly possible".
He said they planned to start using two other airports, at Jacmel in Haiti and San Isidro in the neighbouring Dominican Republic.
US Army Maj Gen Daniel Allyn, the deputy commander for relief operations, said the military had already delivered 400,000 bottles of water and 300,000 food rations.
Earlier, Haiti's president said aid delivery was the main problem.
Help came "very fast," Rene Preval said. "When it arrives, the question is: where are the trucks to transport it, where are the depots?"
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