People wail from the rubble across Port-au-Prince, where a hospital reportedly collapsed and bodies lie in the streets. One aid official estimates 'there must be thousands of people dead.'

Reporting from Mexico City - A mighty earthquake rocked the tiny, impoverished island nation of Haiti today, collapsing a hospital, the presidential palace and other buildings and triggering what one diplomat called a "catastrophe."

As night fell on the Haitian capital of Port-au-Prince and other towns, reports of extensive destruction were trickling out. Tsunami alerts were issued for Cuba, the Bahamas and much of the Caribbean.

The quake, one of the most powerful ever in the region -- measuring a preliminary magnitude of 7.0 and centered about 10 miles west of Port-au-Prince, a city of 2 million -- had a shallow depth of just five miles. It struck at 4:53 p.m., followed by several strong aftershocks.

All of that augured vast damage and overwhelming casualties. Electricity was out tonight through the darkened capital, phone lines were down, and the airport was shut. Screams for help seeped from felled buildings, and chaos reigned.

"I can hear very distressed people . . . a lot of distress, people wailing, trying to find loved ones trapped under the rubble," Ian Rodgers, with Save the Children in Port-au-Prince, told CNN by telephone.

In Washington, President Obama pledged to help the crippled country.

Secretary of State Hillary Rodham Clinton, in remarks before a speech in Hawaii, said the U.S. was assessing the situation and "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," Clinton said. "And our prayers are with the people who have suffered, their families and their loved ones."

Philip J. Crowley, the State Department's senior spokesman, said U.S. officials were meeting to plan "significant" assistance to Haiti.

He said U.S. Embassy personnel have reported widespread damage, including collapsed buildings and walls, and bodies lying in the streets. The presidential palace, a graceful white French colonial structure visited by President Clinton in 1994, has sustained heavy damage, Crowley said.

U.S. officials plan to send teams to assess Haiti's needs, but they first needed to determine whether airport runways were able to receive cargo planes carrying aid, Crowley said.

A spokesman for the U.S. Southern Command, which oversees American military operations in the Caribbean and South America, said officials are assessing what assistance or aid might be needed.

"We are monitoring the situation and staying in close contact with the State Department," said Jose Ruiz, a spokesman for the command.

The Associated Press said its reporters saw a hospital collapse in the wealthy suburb of Petionville that overlooks the capital.

A spokeswoman for Catholic Relief Services said the group's representative in Haiti, Karel Zelenka, described "total disaster and chaos" before the telephone line went dead. Zelenka told colleagues that the Haitian capital was covered in dust.

"He estimates there must be thousands of people dead," the spokeswoman, Sara Fajardo, said in an interview from the group's office in Maryland.

Fajardo said the group has stockpiles of food and other goods to serve 5,000 families but that aid workers are concerned that relief efforts could be impaired by poor road conditions and lack of security.

"Within a minute of the quake . . . soil, dust and smoke rose up over the city, a blanket that completely covered the city and obscured it for about 12 minutes until the atmospheric conditions dissipated the dust," Mike Godfrey, who works as a contractor for USAID, told CNN from Port-au-Prince.

"I think it is really a catastrophe of major proportions," said Haiti's ambassador to the U.S., Raymond Joseph.

People communicating by Twitter said that while they felt the quake in the Haitian city of Cap-Haitien, in the north, there was little damage.

Haiti is the poorest nation in the Western Hemisphere and one of the poorest in the world. Already battered by storms, military coups and gang violence, much of Haiti is a hodgepodge of slums, poor construction and people living on the edge.