President Obama said the United States government would offer "swift, coordinated, and aggressive" aid to the estimated three million people caught in the disaster.
"This is a time when we're reminded of the common humanity that we all share," Obama said. "With just a few hundred miles of ocean between us and a long history that binds us together, Haitians are our neighbors in the Americas and here at home."
Secretary of State Hillary Clinton joined in: "It is Biblical, the tragedy that continues to daunt Haiti and the Haitian people. It is so tragic." Clinton was on a trip to talk with leaders of nations in the Pacific Rim, but she cut it short to return to Washington.
Aftershocks continued today, though they were diminishing in strength and frequency.
A radio host named Carel Pedre described what he saw via Twitter, writing, "1st After Shock Of The Day!!! Haiti is still shaking!! HELP!!"
Pedre broke into tears during a phone interview. "I've not had any contact from my mother and my brother and I wonder about what's going to happen and I'm scared," he said.
Another man in Port-au-Prince, Richard Morse, said, "Everyone slept outside last night. People are afraid of remaining inside buildings. As I walked the streets I see bodies.
"People are calm, trying to help each other," he said. "Others are visiting people to see if they are OK. Some are digging bodies out of buildings."
The control tower at the country's main airport is said to have collapsed, hindering rescue efforts, but the runways were usable and the U.S. Air Force sent in equipment to provide air traffic control for relief planes.
Commercial flights were canceled, but American Airlines told ABC News it sent "a couple" of American Eagle commuter jets from San Juan, Puerto Rico, carrying supplies for airline employees.
Haitian-born rap star Wyclef Jean, arguably the most recognizable Haitian in the world, has been urging his fans to donate money to relief efforts through his foundation, Yele Haiti. The 37-year old star said he is on his way to Haiti, a country for which he was made roving ambassador in 2007.
"Haiti has suffered a natural disaster of unprecedented proportions…unlike anything the country has ever experienced," he wrote on his Web site.
The White House said Americans trying to locate family members in Haiti can call the State Department at 888-407-4747. The president said that while Americans may be facing financial trouble at home, those who wanted to donate money to the rescue effort could go to www.whitehouse.gov for more information on how to contribute.
According to the U.N., collapsed buildings include the national penitentiary, where prisoners have escaped, the presidential palace, the Parliament Building, the National Cathedral and at least four other ministry buildings including the finance ministry.
Obama said search-and-rescue teams from Florida, California and Virginia would arrive in Haiti today and tomorrow, with more teams being prepared.
"The rescue and recovery effort will be complex and challenging," he said. "The first hours and days are absolutely critical to saving lives and avoiding greater tragedy."