"We've seen this with almost all large, shallow earthquakes," said Don Blakeman of the National Earthquake Information Center in a telephone interview with ABC News. "We're going to have an aftershock series, and as time passes, the size of the aftershocks and the frequency will decrease."
The White House issued a statement tonight from President Obama: "My thoughts and prayers go out to those who have been affected by this earthquake. We are closely monitoring the situation and we stand ready to assist the people of Haiti."
U.S. military and humanitarian services said they were ready to offer help, but officials said privately that they had to wait for a formal request from the Haitian government.
"The United States 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 and our prayers are with the people who have suffered, their families and their loved ones," Secretary of State Hillary Clinton said in Honolulu, Hawaii, where she was to make a speech on Asian relations.
"We are standing by to help in any way we can," said Crowley.
The U.S. Agency for International Development asked California Task Force 2, the Los Angeles County Fire Departent's search and rescue team, and Virginia Task Force 1, sponsored by the Fairfax County, Va., Fire and Rescue Department, to prepare to deploy to Haiti to help with the recovery effort, should the country request assistance.
The Coast Guard has said that it will send out reconnaissance planes at first light tomorrow to assess the damage to the region. They also have four cutters in the area should a request be made for assistance.
Efforts to get aid into Haiti could be hampered by damage to the airport in Port-au-Prince. The Federal Aviation Administration reported that the air traffic control tower at the airport has collapsed.
Flights were being routed around Haitian air space Tuesday night because no air traffic control services were being provided.
"Everybody is just totally, totally freaked out and shaken," said Henry Bahn, a visiting official with the U.S. Department of Agriculture. "The sky is just gray with dust."
Bahn said he was walking to his hotel room when the ground began to shake.
"I just held on and bounced across the wall," he said. "I just hear a tremendous amount of noise and shouting and screaming in the distance."
In the United States, Rose Leandre was anxiously trying to get through to any family members in her native Haiti on Tuesday evening.
The Spring Valley, N.Y., woman runs the Haitian-American Cultural and Social Organization, which provides immigration, social and educational outreach to the sizable Haitian community --- estimated to be at least 11,000, according to U.S. Census figures -- in Rockland County, one of New York City's northern surburbs.
"Everybody's been calling all over and calling each other, 'have you heard from your family?' And so far, no one's getting through … so far, within the community, no one has gotten through to anyone in Haiti," Leandre said.
Leandre added that she was desperate to make contact with her mother, who traveled to Haiti, to an area 20 minutes outside Port-au-Prince, just Monday.