A major earthquake rocked Haiti, killing possibly thousands of people as it toppled the presidential palace and hillside shanties alike and left the Caribbean nation appealing for international help.

A five-story U.N. building was also brought down by Tuesday's 7.0 magnitude quake, the most powerful to hit Haiti in more than 200 years according to the U.S. Geological Survey.

Reuters television footage from the capital, Port-au-Prince, showed scenes of chaos on the streets with people sobbing and appearing dazed amid the rubble.

The quake's epicenter was only 10 miles from Port-au-Prince. About 4 million people live in the city and surrounding area. Aftershocks as powerful as 5.9 rattled the city throughout the night and into Wednesday.

Reports on casualties and damage were slow to emerge due to communication problems but Brazilian General Carlos Barcellos said at least four Brazilian members of the U.N. peacekeeping mission in Haiti were killed. A large number of Brazilian soldiers were also missing.

The poorest country in the Western Hemisphere, Haiti is ill-equipped to respond to such a disaster, lacking heavy equipment to move debris and a sufficient emergency personnel.

I am appealing to the world, especially the United States, to do what they did for us back in 2008 when four hurricanes hit Haiti, Raymond Alcide Joseph, Haiti's ambassador to Washington, said in a CNN interview.

At that time the U.S. dispatched ... a hospital ship off the coast of Haiti. I hope that will be done again ... and help us in this dire situation that we find ourselves in. I'm asking the Haitians who are abroad to work together and bring all the effort in a concerted manner to help those back home.

Sara Fajardo, a spokeswoman for Catholic Relief Services, told the Los Angeles Times its representative in Haiti said the death toll could be in the thousands.

The Medecins Sans Frontieres (Doctors Without Borders) aid organization said it was treating about 600 people in its hospitals in Haiti. It also was sending reinforcements to the disaster zone, as was the International Red Cross.

U.S. ORGANIZING RESPONSE

U.S. President Barack Obama said his thoughts and prayers were with the people of Haiti and pledged immediate aid. He was to make a statement on the quake on Wednesday but senior administration officials said a time had not been set.

A late-night White House meeting involving various arms of the government took place to coordinate the U.S. response. The State Department urged Americans not to travel to Haiti.

In Geneva, U.N. officials said they expected the world body would issue an international emergency appeal for funds and other assistance for Haiti in the next few days, once needs on the ground had been assessed.

The Inter-American Development Bank said it would provide $200,000 in immediate aid. The World Bank, which said its local offices were destroyed but most staff were safe, planned to send a team to help assess damage and plan a recovery.

The U.S. Coast Guard said it was sending cutters and aircraft close to Haiti to give humanitarian assistance.

French President Nicolas Sarkozy's office said in a statement on Wednesday France was sending rescue services to help operations and find French citizens.

The quake hit at 5 p.m. (2200 GMT), and witnesses reported people screaming Jesus, Jesus running into the streets as offices, hotels, houses and shops collapsed. Experts said the quake's epicenter was very shallow at a depth of only 6.2 miles, which was likely to have magnified the destruction.

The presidential palace lay in ruins, its domes fallen on top of flattened walls. CNN reported on its website that Haitian Ambassador Joseph said President Rene Preval was safe, but gave no further details.

Bloodied and dazed survivors gathered in the open and corpses were pinned by debris.

The United Nations said a large number of its personnel were unaccounted for after a five-story building at its headquarters collapsed.

The whole city is in darkness. You have thousands of people sitting in the streets with nowhere to go, said Rachmani Domersant, an operations manager with the Food for the Poor charity. There are people running, crying, screaming.

LITTLE HELP FOR VICTIMS

In the hillside neighborhood of Petionville, Domersant said he saw no police or rescue vehicles.

People are trying to dig victims out with flashlights, he said. I think hundreds of casualties would be a serious understatement.

Witnesses said they saw homes and shanties built on hillsides tumble as the earth shook.

The car was bouncing off the ground, Domersant said.

U.N. officials said normal communications had been cut off and the only way to talk with people on the ground was via satellite phone. Roads were blocked by rubble.

U.N. peacekeeping chief Alain Le Roy said the main U.N. building in Port-au-Prince had collapsed. We don't know how many people were in the building, he told reporters.

Some 9,000 U.N. police and troops are stationed there to maintain order and many countries were trying to determine the welfare of their personnel.

France's minister for cooperation, Alain Joyandet, said on French radio the Hotel Montana had collapsed and that about 100 of its 300 guests had been evacuated.

Le Roy's deputy Edmond Mulet said 200 to 250 people worked in the collapsed U.N. building during normal hours.

(Additional reporting by Alister Bull, Jane Sutton and Phil Barbara; Writing by Pascal Fletcher; Editing by Alan Elsner)

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