The State Department had managed to contact the US deputy chief of mission in Port-au-Prince, David Lindwall who said the embassy had not been damaged, a US State Department spokesman said.
"He has seen walls down, a number of people injured and killed," Philip Crowley told reporters in Washington.
Crowley said that embassy officials were reporting that buildings had collapsed and had seen a number of bodies that had been hit by debris.
"So clearly, there's going to be serious loss of life in this," he said.
US Secretary of State Hillary Clinton called it a "catastrophic earthquake" in remarks made during her journey to Asia.
The 7-magnitude earthquake struck outside the capital Port-au-Prince around dusk.
With the city in darkness, the death toll and extent of damage were difficult to determine. Crowley said an evaluation would be made first thing in the morning.
"One of the questions will be at first light, what's the condition of the airport and at what point can we begin to put teams on airplanes and get them down there," Crowley told reporters.
He said that relief teams were being assembled in Fairfax and Los Angeles which will work closely with US Agency for International Development (USAID).
A task force would be "working on this issue through the night", Crowley said.
Crowley said communications within Haiti were difficult because landlines and cellphone towers had likely been disturbed by the quake. He said embassy officials had not yet been able to determine the safety of all employees.