On the ground in Haiti with survivors as they desperately plead for help
The US is sending up to 3,500 soldiers and 2,200 marines to Haiti to help rescue efforts in the wake of the devastating earthquake.
President Barack Obama pledged one of the biggest relief efforts in recent US history and said Haiti would "not be forgotten" in its hour of need.
The search for survivors continues but rescuers lack heavy lifting equipment and many are using their bare hands.
The Red Cross estimates 45,000-50,000 people are dead and up to 3m affected.
BBC correspondents say the situation is increasingly desperate, with aid only trickling in.
President Obama: 'Search and rescue teams are actively working to save lives'
Mr Obama confirmed that some US rescuers were already working on the ground in Haiti.
Speaking in Washington, he promised the country "every element of our national capacity, our diplomacy, and development assistance, the power of our military and most importantly, the compassion of our country" following the disaster.
"To the people of Haiti, we say clearly and with conviction, you will not be forsaken, you will not be forgotten," he said.
However he warned it would take time for much-needed help to reach people.
Mr Obama also promised an immediate $100m for Haiti's relief effort and said that investment would grow over the coming year to aid long-term recovery.
AT THE SCENE
Andy Gallacher, BBC News, Haiti
Haiti is at a critical juncture at the moment.
I've come across two schools that have completely collapsed, you could see the bodies trapped inside - but there were no rescue teams on the ground. I haven't seen anyone in the two days I've been here.
Haitians are still digging through the rubble with their hands. The bodies are beginning to build up, both on the streets and in public spaces.
There are no coffins here, no arrangements for burials. There is no sense that the promised relief efforts have begun in earnest.
The first 100-strong contingent from the US Army's 82nd Airborne Division is expected to arrive in Haiti by the end of Thursday, with several hundred more due by Friday.
The USS Carl Vinson aircraft carrier will arrive on Thursday. The USS Bataan, carrying a marine expeditionary unit, is also on its way.
The US forces will join Haitians and international search and rescue teams already on the ground.
Aid groups say it is a race against time to find people trapped under the rubble of the collapsed buildings.
Elisabeth Byrs, of the UN's Office for the Coordination of Humanitarian Affairs, said: "The priority is to find survivors. We are working against the clock."
The head of Medecins du Monde, Olivier Bernard, told AFP news agency that aid had to arrive by Thursday evening.
We just don't know what to do. You can see how terrible the damage is. We have not been able to get into all the areas
"To save lives, surgery must be available ideally within the first 48 hours," he said.
Doctor's assistant Jimitre Coquillon told Associated Press: "This is much worse than a hurricane. There's no water. There's nothing. Thirsty people are going to die."
The BBC's Matthew Price in Port-au-Prince says Haiti is in massive need of food, water and medicine, as well as bulldozers and heavy lifting equipment.
But perhaps more than anything it needs someone to take charge here, our correspondent says. The government is fragile at the best of times and there is no sense it is able to do anything for now.
Meanwhile, the US Federal Aviation Authority said it had stopped civilian flights to Haiti at the Haitian government's request because there was not enough space on the ground for more planes and only limited fuel for them to leave. Flights were later resumed, reports said.
The director of Port-au-Prince's general hospital said that by 1100 (1600 GMT) at least 1,500 bodies were already stacked inside and outside the morgue, with police continuing to bring more corpses on pick-up trucks, Reuters reports.
UN Secretary General Ban Ki-moon said it could be days before even an estimate of the death toll from the earthquake could be made, but said he feared it would be "very high".
Ban Ki-moon: "Haiti will need every ounce of help we can offer"
Up to 150 UN staff remain unaccounted for following the collapse of the UN headquarters building in Port-au-Prince and 36 UN military and police personnel are now confirmed dead.
On a note of hope, he recounted the survival of an Estonian UN official who was detected under 4m (13ft) of rubble after scratching noises were heard. He was dug out and is now in hospital.
A few US aid planes and a 50-strong Chinese rescue team with sniffer dogs have landed at the airport serving the capital, Port-au-Prince.
Cuba already had more than 300 doctors in Haiti before the earthquake and they have been treating the injured in field hospitals.
Other plane-loads of rescuers and relief supplies are on the way from the EU, Canada, Russia and Latin American nations.
A British rescue team with heavy lifting gear and dogs has now arrived in Haiti.
UK Prime Minister Gordon Brown said: "The last 24 hours have been truly horrific for the people of Haiti. It's a catastrophe that's still unravelling, it's a tragedy beyond imagination."
US Secretary of State Hillary Clinton cancelled a trip to Asia to deal with the crisis. She said there were tens of thousands of casualties in Haiti and that tens of thousands of buildings had collapsed.
"This is going to be a long-term effort," she said.
Her husband, Bill Clinton, the UN special envoy for Haiti, told the Washington Post the quake was "one of the great humanitarian emergencies in the history of the Americas".
Meanwhile, French President Nicolas Sarkozy called for an international conference on rebuilding Haiti to be held with the help of nations including the US, Brazil and Canada.
"Haiti must not remain a battered country," he said.
The World Bank is funding $100m of emergency aid. The World Food Programme is working on supplying 15,000 tonnes of food and the Red Cross has begun a $10m appeal.
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