PORT-AU-PRINCE -- The top U.N. envoy was missing, while his headquarters and Haiti's historic presidential palace lay in ruins Wednesday after a powerful 7.0-earthquake crippled the nation's communications with the outside world and left uncounted casualties.

Government and private aid agencies were poised to descend on the country even as the White House and United Nations conferred on how to proceed. The U.N. said late Tuesday that "a large number of personnel remain unaccounted for" after its headquarters suffered severe damage.

Sources told The Miami Herald that Hédi Annabi, head of the U.N. stabilization force, and his deputy were among the missing.

"As far as casualties, we can't say for now, " said U.N. spokesman Ari Gaitanis.

A hospital was reported to have collapsed and people were heard screaming for help, and portions of the National Palace in downtown Port-au-Prince crumbled.

"There are people injured in the palace, " Fritz Longchamp, the building's executive director, told The Miami Herald. "I'm calling for help and medical assistance for them."

Haitian President René Préval, who was not in the palace at the time of the quake, sought safe haven on the island, which is still rebounding from recent devastating hurricanes.

Part of the road to Canape Vert, a suburb of the capital, has collapsed, along with houses perched in the mountains of Petionville, where the quake was centered. Petionville is a suburb about 10 miles from downtown Port-au-Prince.

More than 20 aftershocks followed the main 4:53 p.m. earthquake, according to the National Oceanic and Atmospheric Administration. A tsunami alert was briefly issued for the region and canceled.

Eyewitness accounts of the destruction were hard to come by. Some came via Twitter, Facebook and Skype. Richard Morse, owner of the Oloffson Hotel in Port-au-Prince, sent tweets to the outside world.

"Just about all the lights are out in Port au Prince, " he said. "People still screaming but the noise is dying as darkness sets. Lots of rumors about which buildings were toppled. The Castel Haiti behind the Oloffson is a pile of rubble. It was eight stories high. Our guests are sitting out in the driveway."

Haitian businessman Georges Sassine, who was in Washington, spoke to his wife minutes after the quake.

"She said, suddenly her car started shaking, and she saw houses crumbling and she could not understand what was happening, " he said.

Antwan Edmund, former head of the Caribbean-Central American Action advocacy group, said he was "sitting in Port-au-Prince watching the mountain crumble."

Raymond Alcide Joseph, Haiti's ambassador to the United States, told The Miami Herald that the quake has crippled his country.

It was "a catastrophe of major proportions, " he said.

President Barack Obama was aware of the tragedy, the White House said, and the State Department is working to confirm the safety of staff at the U.S. Embassy in Port-au-Prince.

"My thoughts and prayers go out to those who have been affected by this earthquake, " Obama said in a statement. "We are closely monitoring the situation and we stand ready to assist the people of Haiti."

Former President Bill Clinton, U.N. Special Envoy for Haiti, issued a statement offering assistance.

"My U.N. office and the rest of the U.N. system are monitoring the situation, and we are committed to do whatever we can to assist the people of Haiti in their relief, rebuilding and recovery efforts, " he said.

In Honolulu, Hawaii, Secretary of State Hillary Clinton said America's thoughts were "with the people of Haiti."

Florida Gov. Charlie Crist said the state is prepared to assist. "Our hearts and prayers go out to them all, " he said.

Help was on the way.

The U.S. Agency for International Development is dispatching a Disaster Assistance Response Team and has activated its partners, the Fairfax County Urban Search and Rescue Team and the Los Angeles County Search and Rescue Team. The USAR teams will be composed of up to 72 personnel, six search and rescue canines and up to 48 tons of rescue equipment.

The USAR team will be accompanied by USAID disaster experts who will assist with assessments of the situation.

"This is a tragic situation and we will work alongside the Haitian government to provide immediate assistance in the rescue effort, " said USAID Administrator Rajiv Shah.

In Miami, a prayer service is planned for quake victims at 9 a.m. Wednesday at the Cathedral of St. Mary, 7525 NW Second Ave., and local aid efforts were starting to form.

The University of Miami began assembling an emergency response team to use a private plane to fly to Haiti, said Michel Dodard, an assistant professor and member of the school's medical and community development program in Haiti.

The moment he heard about the earthquake, Dodard contacted his two brothers who live there, one in Petionville -- the center of the quake.

"Clearly, what they are describing is a dreadful situation, " Dodard said. "Haiti has a very fragile disaster relief to begin with and many of the construction is extremely haphazard. You see shantytowns there, and they collapse sometimes during a tropical storm -- not even a hurricane."

The quake rattled the U.S. Navy base at Guantánamo Bay, Cuba, just after 5 p.m. There was no immediate report of damage or injury.

"It felt like when a building shakes when a subway goes by. But I know there's no subway here and the island's not moving, " said Army Maj. Diana R. Haynie.

An American Airlines flight bound for Miami with about 200 passengers aboard -- believed to be the last flight out of Haiti on Tuesday -- arrived at Miami International Airport at 8:42 p.m. The jet had been preparing for takeoff at the time of the earthquake. It was allowed to leave after airport personnel determined the runway was not seriously damaged.

"Everything was chaotic, " said Jarrod Seth of Seattle, who had traveled to Haiti with his wife, Sena, to adopt two children. He had just checked in at the Port-au-Prince airport when the quake struck. "People were falling all over each other, ceiling tiles came down, windows crashed. It was the scariest thing."

At Southern Command in Miami-Dade, the military was on standby for a formal request from the State Department to provide assistance. None had been made.

South Florida Haitians dialed friends and relatives in the island nation -- to no avail. All connections were cut.

"My mother just went to Haiti on Friday and I'm terrified. " said Gepsie Metellus, a Haitian community leader.