Secretary of State Hillary Rodham Clinton is heading to Haiti to inspect earthquake damage and confer with Haitian and other officials on how to speed humanitarian aid to thousands of sufferers and further shape the recovery effort.

At the White House, President Barack Obama scheduled an Oval Office meeting Saturday with the two former presidents who have agreed to spearhead private fundraising efforts for Haiti {mdash} Republican George W. Bush and Democrat Bill Clinton.

Hillary Clinton will be the highest-ranking U.S. official to visit Haiti since Tuesday’s devastating earthquake, which the Red Cross estimates killed 45,000 to 50,000 people.

The White House has said Obama has no immediate plans to visit himself.

Obama spoke with Haitian President Rene Preval on Friday - two previous attempts to do so had failed because of interference with communications - and pledged that the U.S. would stand with it for the immediate search-and-rescue effort and for the eventual rebuilding.

Preval was among the Haitian officials Clinton planned to meet with on Saturday.

“There are going to be many difficult days ahead,” Obama said at the White House.

The administration on Friday also acknowledged the limits of its initial relief efforts and promised a quick speed-up in the delivery of water and other badly needed supplies.

The State Department raised the confirmed U.S. death toll to six, and said 15 other Americans are presumed to have died.

Dr. Rajiv Shah, the White House’s designated coordinator of the U.S. relief effort, told reporters at the State Department on Friday that the main focus of U.S. efforts was still on recovering trapped survivors. Shah was to accompany Clinton to Haiti.

Shah, who just last week became administrator of the U.S. Agency for International Development, indicated that relief supplies will begin flowing more freely in the next few days. He said he had pulled together $48 million worth of food assistance, enough to feed the affected 2 million Haitians for several months. He also said 100,000 10-liter containers of water would be there soon.

But he and others also stressed the physical and logistical limits on the U.S. ability to deliver the aid.

State Department spokesman P.J. Crowley said the major obstacle was the inability to use the main port in Port-au-Prince, the capital, because of extensive damage. There also is only one airport at which to receive shipments of food and other supplies.

Crowley said Friday’s arrival of the aircraft carrier USS Carl Vinson, with 19 helicopters on board, was like suddenly having “a second airport.” Helicopters immediately began ferrying water and other supplies into Haiti.

At the Pentagon, Defense Secretary Robert Gates also noted the constraints on carrying out a full relief effort. He said that while dropping supplies from the air might seem efficient doing so could backfire and cause rioting as people fight over the supplies.

About 4,200 U.S. military personnel were operating within Haiti or from Navy and Coast vessels offshore, U.S. Southern Command said in an update Friday night.

An additional 6,300 personnel are scheduled to arrive by Monday to help distribute aid and prevent potential rioting among desperate survivors.

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