Naval ships steamed south and flights began shuttling search-and-rescue teams to dig through rubble in Port-au-Prince. Military aircraft flew over the island, mapping the destruction, while U.S. officials coordinated the efforts of nongovernmental aid agencies. Coast Guard helicopters began flying seriously wounded Americans from the U.S. Embassy on the island nation to the U.S. military base at Guantanamo Bay, Cuba, about 200 miles away.

"With just a few hundred miles of ocean between us and a long history that binds us together, Haitians are neighbors of the Americas and here at home," Obama said, calling the earthquake an "especially cruel and incomprehensible tragedy."

The U.S. government's response accelerated Wednesday as the extent of the disaster became clear. Obama canceled a speech on job creation as his top advisers huddled in the White House Situation Room throughout the day.

But even as U.S. agencies lined up to help, officials sounded a note of concern, saying they are deeply worried about whether Haiti's infrastructure can handle the influx of help. The island's airport and seaport sustained substantial damage in the temblor.

"If the port is severely damaged, that makes it very, very difficult" to deliver relief supplies, said U.S. Coast Guard Rear Adm. James A. "Jim" Watson IV, director of Atlantic area operations.

Charities large and small also mobilized Wednesday to help. World Vision staff members worked to move blankets, bottled water and other relief supplies. The American Red Cross promised tarps, mosquito nets and cooking sets for 5,000 families from a warehouse in Panama. And churches and small nonprofits called in volunteers and collected canned goods.

Response from donors was swift.

Charities set up Twitter alerts, Facebook groups and text-message donation numbers. By early Wednesday evening, more than $1 million had been raised for the Red Cross, $10 at a time, by people texting "Haiti" to 90999. The World Bank announced $100 million in emergency grant funding. Ted Turner, who created the U.N. Foundation, announced a $1 million commitment to the relief effort. U.N. Secretary General Ban Ki-moon announced $10 million.

Secretary of State Hillary Rodham Clinton and Defense Secretary Robert M. Gates canceled trips to Australia to help coordinate the U.S. response in the Caribbean. Clinton, who honeymooned on the island of Hispaniola decades ago and whose husband, Bill Clinton, is the U.N. special envoy to Haiti, was visibly shaken as she spoke in Hawaii about the devastation.

"It is biblical, the tragedy that continues to stalk Haiti and the Haitian people," she said, adding that the recovery from four hurricanes in 2008 had begun to take hold when "along comes Mother Nature and just flattens the whole place." Clinton, who had planned to go on to the South Pacific, will now return to Washington immediately, aides say.

U.S. officials expressed particular concern about the 172 Americans posted at the embassy, and about the roughly 45,000 Americans who live in Haiti, few of whom had been in contact with officials by midday. The embassy building is one of a handful that were relatively unscathed, officials said, and is serving as the center of the U.S. relief effort.

"We've received a number of reports of injured U.S. citizens," Cheryl D. Mills, the chief of staff at the State Department, told reporters. Almost all of the embassy workers have been accounted for, she said.

Rajiv Shah, administrator of the U.S. Agency for International Development and the overall coordinator of U.S. relief efforts for Haiti, said a 15-member Disaster Assistance Response Team was to be in the country by Wednesday evening to conduct surveillance that can guide governmental and private relief efforts.

Those teams will be quickly followed by two 72-member urban rescue units from Fairfax County and Los Angeles. Another crisis team from Miami also mobilized Wednesday, to be followed by more search-and-rescue operations put together by the Federal Emergency Management Agency, Shah said.

Military officials at the U.S. Southern Command said they began moving ships toward Haiti as they braced for a long recovery.

Officials ordered the hospital ship USNS Comfort -- which aided Haiti after hurricanes struck Port-au-Prince two years ago -- to dock off the coast and assist the sick and wounded. The aircraft carrier USS Carl Vinson, based at Norfolk, is scheduled to arrive in Haiti on Thursday afternoon, said Gen. Douglas M. Fraser, the head of Southern Command.

Fraser said the carrier has less than its full complement of sailors, Marines and equipment, but it will take on helicopters and provisions as it heads south.

The Navy also ordered the amphibious assault ship USS Bataan to Haiti with about 2,000 Marines to help the country maintain security.

Military officials said their first mission will be setting up a temporary air traffic control system to replace the damaged tower at the nation's airport. Coast Guard operations were focused on establishing broad sea-lift capacity at Port-au-Prince's seaport.

Four Coast Guard C-130 aircraft will be made available to State Department workers and others for logistical needs, including evacuations, Watson said. The service was also prepared to move two more cutters, the Tahoma from Guantanamo Bay, and the Vigorous, a three-day trip from South Florida, to the area.

White House officials warned against people or groups trying to enter the country in an effort to lend a hand. Given the fluid situation in Haiti, it is not "helpful to have massive waves of uncoordinated relief arriving," White House official Patrick Gaspard -- the most prominent Haitian American in the Obama administration -- said in a conference call with organizations.