WASHINGTON — President Barack Obama on Wednesday promised an all-out rescue and humanitarian effort to help the people of Haiti overcome a "cruel and incomprehensible" tragedy, the ruinous earthquake that ravaged the poorest country in the Western Hemisphere.

Obama said U.S. civilian disaster assistance and search and rescue teams would be arriving Wednesday and Thursday. The damage to the Caribbean country was assessed from above during military overflights, the president said.

The military's Southern Command said it was sending Navy ships to Haiti and dispatching Air Force specialists to help operate the Port au Prince airport.

Obama sought to show a swift and united disaster response with the United States as an assertive leader, but he said the effort must be an international one, calling upon all nations to join in helping stricken Haitians.

"We are reminded of the common humanity that we all share," Obama said in a brief statement at the White House.

"We have to be there for them in their hour of need."

Obama adjusted his Wednesday schedule, canceling a jobs event in Maryland to better monitor the situation in Haiti.

The 7.0-magnitude earthquake caused thousands of buildings to collapse in Haiti's capital, Port-au-Prince, trapping untold numbers.

Obama said he named U.S. Agency for International Development administrator Rajiv Shah to coordinate American efforts. The White House said Obama told Shah "he expects an aggressive and highly coordinated relief effort."

The first Disaster Assistance Response Team from USAID is expected to arrive in Haiti from San Jose, Costa Rica around 1:30 p.m. (1830 GMT), with a search and rescue team arriving less than two hours later from the Washington D.C. area.

Elements of the U.S. Air Force 1st Special Operations Wing are expected to arrive Wednesday afternoon in Haiti to help with air traffic control and airfield operations at the international airport at Port au Prince.

The U.S. Navy aircraft carrier, USS Carl Vinson, is under way and expected to arrive off the coast of Haiti Thursday. Additional U.S. Navy ships are under way to Haiti, a statement from Southern Command said without more specifics.

A U.S. defense official said early Wednesday that officials were working to try to get the Navy hospital ship USNS Comfort to the area, though it was unclear whether it had been officially ordered to go. It would take some days for the ship, in port in Baltimore, to be serviced, supplied and arrive at Haiti, one official said. The official spoke on condition of anonymity because plans were still being formed then.

Haiti's Ambassador to the U.S. Raymond Joseph told ABC television Wednesday that "a hospital ship off the coast of Haiti is a must for us right now."

The U.S. embassy in Port-au-Prince has no confirmed reports of American citizen deaths. There are an estimated 40,000-45,000 Americans living in Haiti, but exact numbers are difficult to gauge because people come and go.

At dawn Wednesday, a U.S. Coast Guard helicopter evacuated four critically injured U.S. Embassy staff to the hospital on the U.S. Naval base at Guantanamo Bay, Cuba, where the military has been detaining terror suspects for the last seven years.

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." (AP)