Haiti Press Network said the number of dead was unknown hours after the earthquake. It reported the quake, which measured 7.0 on the Richter scale, brought down thousands of houses as well as government buildings and hospitals.
Children were reportedly trapped in schools that came down on top of them.
The earthquake, with an epicenter 10 miles southwest of Port au Prince, struck just before 5 p.m., the U.S. Geological Survey reported. It was followed by dozens of strong aftershocks.
Frank Williams, Haitian director of World Vision International, told CNN "walls from buildings and private residences (were) falling into the streets." Williams said his relief agency's building shook for about 35 seconds and "portions of things on the building fell off."
The Miami Herald said Haitian President Rene Preval has sought safe haven on the island nation.
U.S. Secretary of State Hillary Clinton called the earthquake "catastrophic" as Washington offered aid to victims.
Clinton said the United States was "still gathering information about this catastrophic earthquake, the point of impact, its effect on the people of Haiti."
"The United States is offering our full assistance to Haiti and to others in the region," she said. "We will be providing both civilian and military disaster relief and humanitarian assistance."
The White House said President Barack Obama has directed his staff to make sure U.S. Embassy personnel in Port au Prince are safe and to initiate planning to respond in case humanitarian assistance is necessary.
"My thoughts and prayers go out to those who have been affected by this earthquake," Obama said in a statement released by the White House. "We are closely monitoring the situation and we stand ready to assist the people of Haiti."
The State Department, USAID and the U.S. Southern Command had begun work on coordinating U.S. assistance to Haiti, the White House said.
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