Port-au-Prince, Haiti (CNN) -- Hundreds of thousands of people have died in Haiti's earthquake, the prime minister told CNN Wednesday.
Haitian authorities said the powerful quake destroyed most of the capital city of Port-au-Prince.
A top envoy called it a "major catastrophe."
Haiti's first lady, Elisabeth Debrosse Delatour, reported that "most of Port-au-Prince is destroyed" and that many government buildings had collapsed, Haiti's ambassador to the United States, Raymond Joseph, told CNN Wednesday morning. Delatour said President René Préval was all right, Joseph reported.
Rescue crews were racing Wednesday morning to fully assess the damage in the teeming hillside city, where toppled buildings killed and injured an untold number of people and trapped others in the rubble.
The U.S. State Department has been told to expect "serious loss of life," spokesman P.J. Crowley said, though precise casualty estimates were not immediately available.
About 3 million people -- one-third of Haiti's population -- were affected by the quake, the Red Cross estimated.
Edmond Mulet, the U.N. assistant secretary-general for peacekeeping operations, told CNN that the National Penitentiary collapsed and the inmates escaped, prompting worries about looting by escapees.
President Obama pledged Wednesday that the U.S. government would lead "a swift, coordinated and aggressive effort to save lives" in Haiti after the earthquake.
"For a country and a people who are no strangers to hardship and suffering, this tragedy seems especially cruel and incomprehensible," he said.
The president noted that "military overflights have assessed the damage" from the earthquake and that civilian disaster assistance team were beginning to arrive in Haiti.
Obama said U.S. relief efforts are currently focused on a quick accounting of U.S. Embassy personnel and their families in Port-au-Prince, as well as other American citizens living and working in Haiti. He urged Americans trying to locate family members in Haiti to telephone the State Department.
The main airport in Haiti appears to be operable. U.S. Embassy staff at the airport said the towe