WASHINGTON DC, United States - Rescue teams, doctors and soldiers rushed by land, sea and air Thursday to help drag survivors of Haiti's devastating earthquake from the rubble and bring its people life-saving food and medicines.

US President Barack Obama promised Haitians they would not be forgotten, offering 100 million dollars and every element of US power to help them.

Nations from all corners of the world also joined the vast relief operation, as hundreds of thousands of homeless, injured and traumatized awaited help on the corpse-strewn streets and sidewalks of Port-au-Prince.

Planes began arriving at the capital's still-functioning airport bringing surgeons, field hospitals, water and emergency medical supplies, while search and rescue teams with sniffer dogs readied to pick through the debris.

Governments and aid organisations unlocked millions of dollars and launched appeals for more to help survivors and reconstruct ruined homes, schools and hospitals in one of the world's poorest nations.

Haiti's prime minister said Tuesday's 7.0-magnitude quake, which flattened Port-au-Prince, may have killed more than 100,000 people.

As a wealthy neighbor with the world's most powerful military, the United States was well-placed to lead the effort.

"To the people of Haiti, we say clearly and with conviction, you will not be foresaken, you will not be forgotten," Obama said, offering "every element of our national capacity, our diplomacy, and development assistance, the power of our military and most importantly, the compassion of our country."

Secretary of State Hillary Clinton said the disaster was "beyond our comprehension."

Former US president Bill Clinton, now UN special envoy to Haiti, appealed for funds for what he said was "one of the great humanitarian emergencies in the history of the Americas."

A US Army spokesman said the first members of a brigade of 3,500 US troops will begin arriving in Haiti Thursday, while the aircraft carrier Carl Vinson was also set to arrive with destroyers and more Coast Guard ships.

"Things are in motion," army spokesman Gary Tallman said. "We're getting folks there as fast as we can to provide humanitarian assistance."

A Chinese aircraft bringing relief workers and 20 tonnes of aid reached Haiti and was unloaded by UN Chinese soldiers.

Planeloads of rescue teams and relief supplies were also sent from countries including Belgium, Britain, Canada, France, Germany, Italy, the Netherlands, Poland, Portugal, Russia, Spain and Switzerland.

The Red Cross launched a 10-million-dollar appeal for donations, the World Food Programme offered 15,000 tonnes of food and the World Bank pledged an extra 100 million dollars in aid. The International Monetary Fund also offered 100 million dollars, its head Dominique Strauss-Kahn announced.

"We are entering a critical period. There must be massive humanitarian aid arriving this evening," said Olivier Bernard, president of the medical relief agency organisation Medecins du Monde.

Britain said it would donate close to ten million dollars, as a rescue team it sent touched down to help the hunt for survivors. Prime Minister Gordon Brown described the disaster as "a tragedy beyond imagination".

The international Red Cross said that it was sending 40 tonnes of medical supplies while the International Organisation for Migration called for "tents, tents and more tents" to shelter the homeless.

Israel said it was sending two planes with a field hospital and around 220 emergency personnel.

Cuba, which felt the quake, sent 30 doctors to join staff already in Haiti. Brazil said it was providing 10 million dollars in immediate aid, while Peru, Venezuela, Mexico, Colombia, Guatamala and Chile also promised help.

Canada readied two warships, helicopters and planes with supplies, as well as a large relief and rescue force.

From the Asia-Pacific region, Australia pledged nine million dollars while Japan, well used to quakes, pledged five million dollars and offered tents and rescuers. Taiwan, South Korea and New Zealand also offered aid.

The World Health Organization is deploying specialists to help handle mass casualties and corpses, warning of the danger of communicable diseases.