Around three million people need emergency help in the wake of Haiti's devastating earthquake, the Red Cross said today.
Barack Obama vowed "unwavering support". The US President told reporters he had directed his administration to provide swift, coordinated help to save lives.
In the UK, the Prime Minister promised emergency aid for Haiti today after an earthquake there was thought to have cost the lives of thousands of people. Gordon Brown told the Commons: "Because of the devastating earthquake overnight, Haiti has moved to the centre of the world's thoughts and the world's compassion.
"The Government will respond with emergency aid in firefighters, emergency equipment and finance. And we'll give further support to help the people of Haiti recover from this devastating event."
The quake's epicentre was only 10 miles from the capital Port-au-Prince. That, together with Haiti's crumbling infrastructure, still makes it impossible to gauge how many people might be dead or wounded.
"There's probably three million people potentially affected," said a spokesman for the International Federation of Red Cross and Red Crescent societies, basing the estimate on Haiti's population of 10 million and previous experience in earthquake relief.
"Port-au-Prince has been massively impacted," he said. "There are many, many people trapped in the rubble."
The neighbouring Haitian cities of Carrefour and Jacmel may also be heavily damaged.
The organisation said Haiti's disaster relief teams were "completely overwhelmed."
The first airlifts to Haiti were concentrating on search and rescue efforts, setting up makeshift hospitals and delivering food.
The US, Britain, France, Canada, Germany, China, Mexico and Venezuela pledged immediate support in terms of personnel, cash and supplies.
One of the first teams expected to arrive in Haiti was 37 search and rescue specialists from Iceland, along with 10 tons of rescue equipment.
French rescue authorities said 65 rubble-clearing specialists and six sniffer dogs were leaving for Haiti, while Spain was rushing three planes with at least 100 tons of tents, blankets and cooking kits. Israel was sending an elite Army rescue unit of engineers and medics.
Some aid flights from Europe were delayed by heavy snow. A plane with 64 firefighters and rescue dogs was grounded temporarily at Gatwick airport.
Spain said it would provide £3 million, the Netherlands pledged £2 million, Germany, £1 million and China, £600,000.
UN officials were struggling to assess the scale of the disaster amid badly damaged communication networks and were working with aid agency Telecoms Sans Frontieres to get phone lines working again.
There was no electricity in the capital and roads were filled with rubble and debris.
Irish telecommunications company Digicel said it was donating 5 million dollars to aid agencies working in Haiti. The company - a major mobile phone operator in the Caribbean - also was sending specialists there to help repair the damaged phone network.
UN agencies and Red Cross societies were trying to send in teams and aid from their regional hub in Panama, while USAID was mobilising experts and two urban rescue units.
If aid cannot travel over the airport road to the Haitian capital it may be rerouted through the Dominican Republic, said the World Food Programme official. It planned to airlift 86 tons of high-energy biscuits from El Salvador, enough to feed 30,000 people for a week.
The Christian aid organisation World Vision, which has 400 staff in Haiti, said it would immediately distribute the hurricane relief supplies it had stored there.