"In these difficult hours, America stands united," Obama said. "We stand united with the people of Haiti, who have shown such an incredible resilience, and we will help them to recover and to rebuild."
Obama sent Secretary State Hillary Rodham Clinton to the Caribbean country for the first look by a top U.S. official at the devastation. The White House has said Obama had no immediate plans to visit.
The State Department raised the U.S. death toll to 15, including one department employee. Twenty-three Americans were seriously injured in Tuesday's earthquake, and three U.S. government employees were missing.
The Red Cross has estimated that at least 45,000 people were killed.
Hillary Clinton arrived Saturday on a plane delivering aid and was to depart on another carrying evacuees.
Obama and former presidents George W. Bush and Bill Clinton met in the Oval Office for about half an hour to discuss the assignment he gave them: to lead private fundraising efforts for Haitian relief, including immediate needs and the long-term rebuilding effort.
Both men reassured people that money donated through their Web site, http://www.clintonbushhaitifund.org, would be well spent.
Bush said the best thing people can do is to send money.
"I know a lot of people want to send blankets or water. Just send your cash," said Bush, who was visiting the White House for the first time since leaving office in January 2009.
Bill Clinton, who also is the United Nations special envoy to Haiti, spoke of his decades-long appreciation for the country. The Clintons honeymooned there, and a church they sat in 34 years ago is now "a total rubble." He said he had been to hotels that collapsed and had dined with people who were killed. Clinton also helped restore democracy to Haiti during his presidency.
"I believed before this earthquake Haiti had the best chance in my lifetime to escape its history," Bill Clinton said, speaking of the country's history as one of the world's poorest. "I still believe that ... but it's going to take a lot of help and a long time."
In Miami, Vice President Joe Biden and Homeland Security Secretary Janet Napolitano met with Haitian-American leaders before visiting an air base where relief supplies are being flown to Haiti. South Florida has the largest Haitian-American population in the U.S.
"This is personal," Biden said, touching the arm of White House political director Patrick Gaspard, a Haitian-American with family in Haiti.
U.S. officials said more food and water was on the way. There should be 600,000 humanitarian daily rations - basic nutrition packages that provide 2,300 calories - at Haiti's airport by Saturday evening, according to Tim Callaghan, the administration aide helping oversee relief efforts.
Callaghan, who spoke to reporters on a conference call from Haiti, said water purification units arrived Friday night and that officials hope they will produce up to 300,000 liters of water. More water is coming from neighboring Dominican Republic.
National Security Council aide Denis McDonough said on the same telephone call that 180 tons of relief supplies had arrived in Haiti, but he didn't have a good breakdown on how much had been distributed and where.
The Navy hospital ship USNS Comfort left Baltimore on Saturday, headed to Florida to pick up more medical staff before its scheduled arrival near Haiti on Thursday, said Rear Adm. Victor G. Guillory, Navy commander for the U.S. Southern Command.
The Navy also will try to get the wrecked seaport in the capital of Port-au-Prince up and running. Damage to piers and other facilities have interfered with the distribution of aid. The salvage ship Grasp was on its way to the capital with divers and underwater construction teams capable of building temporary piers and other facilities, said Guillory, speaking on a separate conference call.
The Navy currently is moving supplies into Haiti from the aircraft carrier USS Carl Vinson.
About 4,200 U.S. military personnel were operating within Haiti or from Navy and Coast Guard vessels offshore, the U.S. Southern Command said. Another 6,300 personnel were expected to arrive by Monday to help.
Obama said it was important for everyone in Haiti to understand "there is going to be sustained help on the way."
"But what these gentlemen are going to be able to do is when the news media starts seeing its attention drift to other things but there's still enormous needs on the ground, these two gentlemen of extraordinary stature, I think, are going to be able to help ensure that these efforts are sustained," Obama said of Bush and Bill Clinton.
After the Asian tsunami in 2004, Bush asked his father, former President George H.W. Bush, and Bill Clinton to lead the private fundraising effort. The elder Bush and Clinton also raised private money after Hurricane Katrina in 2005.
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Associated Press writer Libby Quaid contributed to this report.
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On the Net:
White House Haitian Relief:
http://www.whitehouse.gov/haitiearthquake
