A massive 7.0-magnitude earthquake has struck the Caribbean nation of Haiti.
The extent of the devastation is still unclear but there are fears thousands of people may have died.
Haiti's worst quake in two centuries hit south of the capital Port-au-Prince on Tuesday, wrecking the presidential palace, UN HQ and other buildings.
A "large number" of UN personnel were reported missing by the organisation. Many people have spent the night outside amid fears of more aftershocks.
The Red Cross says up to three million people have been affected.
Describing the earthquake as a "catastrophe", Haiti's envoy to the US said the cost of the damage could run into billions.
A number of nations, including the US, UK and Venezuela, are gearing up to send aid.
The quake, which struck about 15km (10 miles) south-west of Port-au-Prince, was quickly followed by two strong aftershocks of 5.9 and 5.5 magnitude.
The tremor hit at 1653 (2153 GMT) on Tuesday, the US Geological Survey said. Phone lines to the country failed shortly afterwards.
There is still no official word on casualties and the extent of the devastation is only now becoming clearer with dawn breaking.
China has already indicated in reports in state media that eight of its peacekeepers are buried and feared dead, with another 10 unaccounted for.
I think hundreds of casualties would be a serious understatement
Rachmani Domersant, operations manager, Food for the Poor
The AFP news agency quoted the Jordanian army as saying three of its peacekeepers had been killed and 21 wounded.
The Brazilian army said four of its peacekeepers were killed and a large number were missing.
A French official told AFP about 200 people were missing in the collapsed Hotel Montana, which is popular with tourists.
There have also been some reports of looting overnight.
Rachmani Domersant, an operations manager with the Food for the Poor charity, told Reuters that overnight the capital was in total darkness.
"You have thousands of people sitting in the streets with nowhere to go. There are people running, crying, screaming.
"People are trying to dig victims out with flashlights. I think hundreds of casualties would be a serious understatement."
Earlier, bodies white with dust could be seen piled on the back of a pick-up truck as vehicles tried to ferry the injured to hospital.Haiti is the poorest country in the western hemisphere and has suffered a number of recent disasters, including four hurricanes and storms in 2008 that killed hundreds.
'Thoughts and prayers'
In a statement issued in New York, the UN said that its local HQ in Haiti had "sustained serious damage along with other UN installations" and "a large number" of personnel were missing.
Carel Pedre, TV and radio presenter, Port-au-Prince
I saw a lot of people crying for help, a lot of buildings collapsed, a lot of car damage, a lot of people without help, people bleeding.
I saw a movie theatre, a supermarket, a cybercafe, an apartment building which collapsed.
Now it's dark outside, there is no electricity, all the phone networks are down, so there's no way that people can get in touch with their family and friends.
There are aftershocks every 15 to 20 minutes. They last from three to five seconds. The first shock was really strong, people were falling in the streets and buildings collapsed.
I didn't see any emergency services, the people at the neighbourhood were trying to help each other.
The streets are narrow and there is lot of traffic and everyone is trying to reach family and friends. Traffic now is really difficult. People don't know where to go or where to start.
UN peacekeeping chief Alain Le Roy said it was unclear how many people were in the building.
The head of the UN mission in Haiti, Hedi Annabi, was reported to have been inside and is unaccounted for.
French Foreign Minister Bernard Kouchner said he was believed to be dead.
The UN's stabilisation mission plays a vital role in ensuring security in Haiti.
Raymond Joseph, Haiti's ambassador to the US, said the presidential palace, the tax office, the ministry of commerce and the foreign ministry had all been damaged, but the airport was intact.
He and Haiti's ambassador to Mexico, Robert Manuel, both said that President Rene Preval and his wife had survived the quake.
The World Bank said its local offices were destroyed but most of the staff were accounted for, Reuters reported.
US President Barack Obama said his "thoughts and prayers" were with the people of Haiti and that he expected "an aggressive, coordinated [aid] effort by the US government".
Venezuela says it will send a 50-strong "humanitarian assistance team".
HAITI COUNTRY PROFILE
Half of Caribbean island of Hispaniola
History of violence, instability and dictatorship
Population of 10 million people
Most live on less than $2 a day
Democratic rule restored in 2006
Economy in ruins and unemployment is chronic
UN peacekeepers deployed - foreign aid seen as vital
Massive deforestation has left just 2% forest
Storms and hurricanes in 2008 left almost 800 dead
The Red Cross is dispatching a relief team from Geneva and the UN's World Food Programme is flying in two planes with emergency food aid.
The Inter-American Development Bank said it was immediately approving a $200,000 grant for emergency aid.
The UK said it was mobilising help and was "ready to provide whatever humanitarian assistance may be required".
Canada, Australia, France and a number of Latin American nations have also said they are mobilising their aid response.
Pope Benedict XVI has called for a generous response to the "tragic situation" in Haiti.
'Shouting and screaming'
In the minutes after the quake, Henry Bahn, a visiting official from the US Department of Agriculture, said he had seen houses which had tumbled into a ravine.
"Everybody is just totally, totally freaked out and shaken," said Mr Bahn, who described the sky as "just grey with dust".
He said he had been walking to his hotel room when the ground began to shake.
"I just held on and bounced across the wall," he said. "I just heard a tremendous amount of noise and shouting and screaming in the distance."
Reports on the Twitter message site, which cannot yet be verified by the BBC, expressed the chaos in the wake of the quake.
Tweets from troylivesay spoke of the worst damage being in the Carrefour district, where "many two and three storey buildings did not make it".
In the immediate aftermath of the quake, a tsunami watch was put out for Haiti, Cuba and the Bahamas, but this was later lifted. [via]