The largest earthquake to hit Haiti in more than 200 years rocked the Caribbean nation Tuesday, and Haitian prime minister Jean-Max Bellerive said Wednesday several hundred thousand people may have died.

Haiti's first lady, Elisabeth Debrosse Delatour, said that "most of Port-au-Prince is destroyed." The Haitian ambassador to the United States, Raymond Joseph, called the quake a "major catastrophe."

About 3 million people - one-third of Haiti's population - were affected by the quake, according to Red Cross estimates.

About 10 million people felt shaking from the earthquake, including 2 million who felt severe trembling, the U.S. Geological Survey estimated.

On Wednesday, President Obama announced a "swift, coordinated and aggressive" U.S. response.

"The reports and images that we've seen of collapsed hospitals, crumbled homes and men and women carrying their injured neighbors through the streets are truly heart-wrenching," Obama said.

Communications were widely disrupted Tuesday, making it impossible to get a full picture of damage as powerful aftershocks shook the country.

U.N. assistant secretary-general for peacekeeping operations Edmond Mulet told CNN that the National Penitentiary collapsed and the inmates escaped, prompting worries about looting by escapees.

Built in 1915, the prison was overcrowded. Enlarged to a total capacity of 1,200, it held 3,908 inmates in December, the U.S. State Department has said.

The earthquake had a preliminary magnitude of 7.0 and was centered about 10 miles (15 kilometers) west of Port-au-Prince at a depth of 5 miles (8 kilometers), the U.S. Geological Survey said.

USGS geophysicist Kristin Marano called it the strongest earthquake since 1770 in what is now Haiti. In 1946, a magnitude-8.1 quake struck the Dominican Republic and also shook Haiti, producing a tsunami that killed 1,790 people.