|Haitian police are stretched thin as violence spreads across the capital [EPA]
The US is prepared to "augment" UN and Haitian government forces if they need help with security, but has denied its military has taken charge of the earthquake-devastated Caribbean nation.
Speaking to Al Jazeera, PJ Crowley, a spokesman for the US state department, rejected suggestions said that US military needs were taking priority over the needs of quake survivors.
"The democratically-elected government of Haiti is in charge," he said.
"This is an international effort, the US is not in charge here, the government of Haiti is in charge, the UN is in charge - we're supporting them."
His comments followed criticism from foreign governments and aid groups that the US has prioritised military needs instead of humanitarian ones after taking over operations at the Haitian capital's airport.
On Sunday Hugo Chavez, the Venezuelan president, accused the US military of occupying Haiti and said Daniel Ortega, the Nicaraguan president, was also concerned about the US deployment.
"It appears that the United States is occupying Haiti militarily, taking advantage of the tragedy," Chavez said on his weekly television show.
The Venezuelan leader's comments are an indication of the sensitivity of US soldiers operating in a Caribbean state where they have intervened in the past.
Nearly 13,000 US troops will be sent to the area by Monday and Barack Obama, the US president, has mobilised military reserves to help in the Haiti operation.
The move was "necessary to augment the active armed forces of the United States for the effective conduct of operational missions, including those involving humanitarian assistance, related to relief efforts in Haiti," Obama wrote in a presidential order.
Emphasising that the US was supporting the Haitian government's "vision of rebuilding" the country and following its priorities, Crowley said it was "absolutely not true" that US military planes with troops were being allowed to land at the airport while those carrying aid supplies were not.
"They are bringing in aid, communications gear for the Haitian government so they can begin to operate and function once again," he said.
Crowley added that the US was "prepared to augment UN and Haiti forces if they need help, but for now, they are there for a humanitarian mission only."
But Lieutenant-General Ken Keen, commander of the US military operation in Haiti, told ABC's This Week programme that the military was going to have to play a security role.
"We are here principally for a humanitarian assistance operation, but security is a critical component," he said.
His comments came as Rene Preval, the Haitian president, told reporters on Sunday that 3,500 US troops would be deployed to help UN and Haitian forces restore security in the capital.
|Fights over food and water have broken out among quake survivors [Reuters]
With Haitian police and UN peacekeepers stretched thin and survivors becoming more desperate by the day, reports described growing incidents of violence across the devastated capital.
In some cases survivors fought each other with knives, hammers and rocks in fights over food and water, while police tried to disperse them with gunfire.
At least two people were shot dead on Sunday, witnesses said.
"We have 2,000 police in Port-au-Prince who are severely affected. And 3,000 bandits escaped from prison. This gives you an idea of how bad the situation is," said Preval, who has already given the US military control over the airport.
The US military's Southern Command said on Sunday that some 1,000 US troops had been put on the ground in Haiti, with another 4,000 stationed on vessels offshore.
A further 7,500 troops were set to arrive by Monday.