Haitian President Rene Preval has said thousands of people are feared dead following a huge quake which has devastated the country's capital.

UN chief Ban Ki-moon said the head of the UN mission in Haiti and his deputy were among more than 100 staff missing. The 7.0-magnitude quake, Haiti's worst in two centuries, struck south of Port-au-Prince, on Tuesday. Prime Minister Jean-Max Bellerive told US network CNN he believed more than 100,000 people had died. The Red Cross says up to three million people are affected. The capital's Catholic archbishop is among those reported killed.

A priest at the Saint Jacques Missionary Centre in western France told Monsignor Joseph Serge Miot had been found dead in his office by fellow missionaries. In his first interview since the earthquake, President Preval told he feared thousands of his people had died. Describing the scene in the capital as "unimaginable", he said: "Parliament has collapsed. The tax office has collapsed. Schools have collapsed. Hospitals have collapsed. "There are a lot of schools that have a lot of dead people in them." A number of nations, including the US, UK and Venezuela, are gearing up to send aid.

Speaking in Washington, US President Barack Obama vowed "unwavering support" for Haiti after what he called a "cruel and incomprehensible" disaster. He said he had ordered "a swift, co-ordinated and aggressive effort to save lives" and that the first US rescue teams would arrive later on Wednesday.

The quake, which struck about 15km (10 miles) south-west of Port-au-Prince, was quickly followed by two aftershocks of 5.9 and 5.5 magnitude. The first tremor had hit at 1653 local time (2153 GMT) on Tuesday, the US Geological Survey said. Phone lines to the country failed shortly afterwards. UN officials said at least five people had died when the UN's five-storey headquarters in Port-au-Prince collapsed and that more than 100 staff were unaccounted for and feared to be under the rubble. UN Secretary General Ban Ki-moon confirmed the Tunisian head of the UN mission in Haiti, Hedi Annabi, and his deputy were missing, along with many others.

He said hundreds of people were feared dead and aerial reconnaissance showed Port-au-Prince had been "devastated" by the quake, although other areas were largely unaffected. Stressing a major international relief effort would be needed, Mr Ban said the UN would immediately release $10m (£6.15m) from its emergency response fund. The airport in Port-au-Prince and a UN logistical base are operational, the UN said, allowing aid to start arriving soon. China has already indicated in reports in state media that eight of its peacekeepers are dead, with another 10 unaccounted for.

The news agency quoted the Jordanian army as saying three of its peacekeepers had been killed and 21 wounded. The Brazilian army said 11 of its peacekeepers had been killed and a large number were missing. A French official also told AFP that about 200 people were missing in the collapsed Hotel Montana, which is popular with tourists. A spokesman for medical charity Medecins sans Frontieres (MSF) said it was able only to offer basic care to the "massive influx" of survivors seeking help because all its buildings had been destroyed. "Unfortunately what we are seeing is a large number of patients in critical condition," he said. There were some reports of looting overnight.

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. With communications destroyed by the earthquake, it is not yet possible to confirm the extent of the destruction, although there were reports on Wednesday of many bodies piled in the streets. People in the capital were lifting sheets on bodies to try to identify loved ones.

Damage has also been reported in the towns of Jacmel and Carrefour, near Port-au-Prince. Guido Cornale, a representative of the UN children's agency, Unicef, in Jacmel said it estimated more than one-in-five buildings had been destroyed. Venezuela says it will send a 50-strong "humanitarian assistance team" to Haiti. 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 head of the International Monetary Fund (IMF) said it would co-ordinate with other international agencies to offer help as swiftly as possible. The World Bank also said it was mobilising a team to assess the damage and plan recovery. It said its offices in Port-au-Prince had been destroyed but that most staff were accounted for.

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. Emmet Murphy, who works for a non-governmental organisation in Haiti, told "I was driving through the mountains on my way home to Jacmel when the car started to shake. It was like a very strong wind was blowing and I nearly lost control of the car.

"I drove further and found the road totally blocked by a massive landslide on the road. I just knew that if I had reached that spot five minutes earlier, I would have been killed." Reports on the Twitter message site, which cannot yet be verified, expressed the chaos in the wake of the quake. One writer said: "4 new aftershock in Haiti at 07:27, 07:41, 09:43 and 09:43: magnitude ranging from 4.5 to 5.4."

Blogger Troy Livesay, in Port-au-Prince, wrote: "Thousands of people are currently trapped. To guess at a number would be like guessing at raindrops in the ocean. Precious lives hang in the balance. "When pulled from the rubble there is no place to take them for care. I cannot imagine what the next few weeks and months will be like."