A powerful earthquake of 7.0 magnitude rocked Haiti just before 5 p.m. Eastern time, 10 miles southwest from the highly populated capital of Port-au-Prince, according to the United States Geological Survey, causing widespread damage and panic with the potential for a high number of casualties in the impoverished Caribbean country.

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The New York Times

There were two aftershocks — of 5.5 and 5.9 magnitude — that followed in the last hour, and more were expected, according to David Wald, a seismologist with the survey.

“The main issue here will probably be shaking,” Mr. Wald said, “and this is an area that is particularly vulnerable in terms of construction practice, and with a high population density. There could be a high number of casualties.”

The city has about 2 million people, according to National Geographic.

According to several news reports, a large hospital in the capital had collapsed, and people were screaming in streets full of rubble.

Elsie St. Louis-Accilien, the director of the Haitian Americans United for Progress in Queens, N.Y., said that she was able to reach the director of Ofatma hospital, in Port-au-Prince. “They are trapped inside,” Ms. St. Louis-Accilien said in a telephone interview. “They were pretty shaken, but they were relieved to be alive.”

She said that the director said that there was “a lot of smoke, a lot of dust,” and that her phone has been ringing nonstop. “People are calling me, elected officials are calling, asking what we can do.”

The White House said President Obama was informed of the earthquake at 5:52 p.m. He directed his staff to begin preparations in case humanitarian assistance is needed. The State Department, the United States Agency for International Development and the United States Southern Command began working to coordinate an assessment, aides said.

“My thoughts and prayers go out to those who have been affected by this earthquake,” Mr. Obama said in a statement. “We are closely monitoring the situation and we stand ready to assist the people of Haiti.”

Haiti, by far the poorest country in the Western hemisphere, has been beset by natural disasters for most of its recent history. The island is struck by an annual series of hurricanes and is particularly vulnerable to storm-related disasters because much of its forests have been chopped down and used for fuel, leaving the country with very little tree cover. In one of its hardest hit years, 2004, Haiti was rocked by powerful Hurricane Jeanne, which caused untold destruction and killed 3,000 people.

Since 2008, the island has been struck by at least three severe hurricanes — Gustav, Hanna and Ike — that have wrought nearly a billion dollars worth of damage and killed 800 people. All of this has taken place against the backdrop of food riots, health crises and near constant government instability and upheavals.

Raymond Joseph, Haiti’s ambassador to the United States, said in an interview on CNN that he had little information about the extent of damage but said the suffering inflicted on the was likely to be “catastrophic.”

Mr. Joseph said that the one official he had reached — identified by The Associated Press as President Rene Preval’s chief of staff, Fritz Longchamp — told him that houses had crumbled “on the right side of the street and the left side of the street.”

An Associated Press videographer saw the wrecked hospital in Petionville, a hillside Port-au-Prince district that is home to many diplomats and wealthy Haitians. And a United States government official reported seeing houses that had tumbled into a ravine.

An earthquake of this magnitude has not hit the region in more than 250 years, according to Mr. Wald. Before Tuesday, the most powerful earthquake to hit the region was 6.7 magnitude, in 1984.

Delores Clark, a spokeswoman for the Pacific Tsunami Warning Center, said that scientists were quickly conducting water level measurements and other analyses to determine whether the quake set off a tsunami in other parts of the Caribbean. As of 6 p.m. Eastern, they had not detected one, she said.

Because the fault that likely caused the earthquake is on land, rather than in the water, Mr. Wald said, there was less of a probability of a tsunami. But such an earthquake likely would mean more damage to the city and its surrounding areas, he added.

Haiti sits in a part of the world where the movement and jostling of plates cause a monumental earthquake capable of generating a tsunami and leveling cities about once every 50 years, said Gerard Fryer, a geophysicist with the Pacific Tsunami Warning Center. The last such earthquake — measuring an 8 in magnitude — struck the Dominican Republic in 1946 and generated a tsunami that rocked Haiti.

Mr. Fryer said that the agency would continue taking measurements because Haiti’s geology is so complicated and unpredictable.

“There are all sorts of earthquake mechanisms that are mixed up in that area,” he said. “From what we understand of the mechanism of the earthquake, a tsunami should not have been generated, but nature does give us surprises.”