Obama said the U.S. is coordinating a government-wide response that includes dispatching search-and-rescue teams and other relief.
"The people of Haiti will have the full support of the United States in the urgent effort to rescue those trapped beneath the rubble and to deliver the humanitarian relief of food water and medicine that Haitians will need in the coming days," he said.
Rajiv Shah, USAID administrator, told Fox News the agency is sending two 72-person search-and-rescue teams to Haiti -- one deploying out of Fairfax County, Va.; the other out of Los Angeles County. Shah said finding victims in the rubble will be the main goal for the next 72 hours.
"The United States is offering our full assistance to Haiti and to others in the region. We will be providing both civilian and military disaster relief and humanitarian assistance. And our prayers are with the people who have suffered, their families, and their loved ones," Secretary of State Hillary Clinton said in a written statement.
The administration is urging Americans who want to help to donate cash. Officials are asking people to donate online to organizations like the Red Cross and Mercy Corps, or to text "HAITI" to "90999" on their cell phones to donate $10 to the Red Cross. That donation will be charged to the cell phone bill of anyone who makes it.
The State Department set up a toll-free number to call for information about family members in Haiti: 1-888-407-4747. The department said some callers may receive a recording because of heavy call volume.
The 7.0-magnitude earthquake caused thousands of buildings to collapse in Haiti's capital, Port-au-Prince, trapping untold numbers in tons of rubble. While estimating the number of casualties was impossible, State Department spokesman P.J. Crowley said, "Clearly, there's going to be serious loss of life in this."
A meeting in the White House late Tuesday brought together senior representatives from the State Department, the USAID, the U.S. Mission to the United Nations, the Defense Department, U.S. Southern Command, the Joint Chiefs of Staff, the Homeland Security Department, the Coast Guard, and national security and White House staff to coordinate the government-wide response.
A defense official told Fox News that one of the first missions will be to assess the humanitarian needs in the country by sending reconnaissance aircraft to survey the scene. As part of that mission, officials will try to determine where they can send in aid.
From there, the official said, the United States will send ships stocked with supplies to Haiti.
Coast Guard officials in Miami also mobilized cutters and aircraft to positions near Haiti to offer humanitarian assistance.
Former President Bill Clinton, who is U.N. special envoy for Haiti, said in a statement: "My thoughts and prayers are with the people of Haiti. My U.N. office and the rest of the U.N. system are monitoring the situation, and we are committed to do whatever we can to assist the people of Haiti in their relief, rebuilding and recovery efforts."
Raymond Joseph, the Haitian ambassador to the United States, said Wednesday morning "there is no way of estimating" the casualties.
"I'm quite sure we're going to face a disaster of major proportion," he said in an interview on ABC's "Good Morning America."
Asked what the Haitians most need, Joseph replied that "a hospital ship off the coast of Haiti is a must for us right now." He said that while it's too soon to say how many people perished in Tuesday's 7.0 magnitude quake, "Thank God that it was after hours." The ambassador noted that a lot of employees had left office buildings before the quake struck.
Joseph noted that major government buildings, including the presidential palace, have fallen.
"If a building like the palace, which is very solid, collapsed," he said, "then the devastation is going to be worse because not all of the buildings are up to code in Port au Prince."
The ambassador also said Haiti badly needs help with first-time responders, "and all kinds of good water and clothes, blankets. Anything that would be needed for victims at the outset."
The Associated Press contributed to this report.