A major earthquake struck southern Haiti on Tuesday, knocking down buildings and power lines and inflicting new misery on the Western Hemisphere's poorest nation. Several witnesses reported heavy damage and bodies in the streets of the capital, Port-au-Prince, where concrete-block homes line the steep hillsides leading inland from the waterfront.

No estimate of the dead and wounded was given Tuesday evening, but the U.S. State Department had been told to expect "serious loss of life," spokesman P.J. Crowley told reporters in Washington.

The world body "is still in the process of gathering information on the extent of the damage and the status of U.N. personnel," said Alain Le Roy, the U.N. undersecretary-general for peacekeeping operations.

According to CNN, the magnitude-7.0 quake - the most powerful to hit Haiti in a century - struck shortly before 5 p.m. It was centered about 15 km (10 miles) southwest of Port-au-Prince, the U.S. Geological Survey reported. It could be felt strongly in eastern Cuba, more than 200 miles away. A tsunami watch was posted for Haiti and parts of Cuba, the Dominican Republic and the Bahamas, but the watch was canceled about an hour after the quake.

Because of the earthquake's proximity to the capital, and because the city is densely populated and has poorly constructed housing, "it could cause significant casualties," said Jian Lin, a senior geologist at the Woods Hole Oceanographic Institution in Massachusetts.

Teams from Doctors Without Borders, or Medecins Sans Frontieres (MSF), on the ground said they witnessed significant damage to the organization's medical facilities, injuries to patients and staff, and an influx of wounded toward hospitals in the capital. MSF said its Trinite trauma center hospital, a 60-bed structure and one of the only free-of-charge surgical facilities in Port-au-Prince, was seriously damaged.

The quake was centered about 10 km (six miles) underground, according to the USGS - a depth that can produce severe shaking. At least 18 aftershocks followed, averaging a magnitude of 5.0, according to the U.S. Geological Survey.

American Airlines Flight 1908 took off from Port-au-Prince shortly after the quake struck with a fraction of its passengers, many people choosing to stay behind. Those who arrived in Miami on Tuesday night described fellow passengers as being in a state of shock and an airport that was badly "cracked up." However, the State Department's Crowley said Tuesday night that the U.S. Embassy reported that the airport was in good enough shape for the United States to start sending teams and assistance within a few hours.