US President Barack Obama pledged $100 million for Haiti quake relief and enlisted two former presidents Bill Clinton and George W. Bush to help raise more, vowing to the Haitian people: 'You will not be forsaken.'

Determined to stay on top of the rapidly unfolding humanitarian crisis in the quake zone, Obama told his top aides that responding to the tragedy should be their top priority.

Obama, facing his biggest test of international relief since taking office a year ago, promised an initial US contribution of $100 million and said he had directed his administration to launch 'a swift, co-ordinated and aggressive effort to save lives and support recovery in Haiti.'

For help he turned to former President Bill Clinton, a Democrat who is already a UN special envoy for Haiti, and former President George W. Bush, the Republican who preceded Obama in the White House.

They agreed to a request from Obama to lead private-sector fund-raising efforts, issuing a joint statement expressing deep sadness at the devastation and suffering in Haiti.

'In the days and weeks ahead, we will draw attention to the many ways American citizens and businesses can help meet the urgent needs of the Haitian people,' Bush and Clinton said.

Their effort will be similar to that performed by Clinton and Bush's father, former President George H.W. Bush, when they led an international relief effort to help the recovery from the 2004 tsunami that swept South Asia and killed 226,000 in 13 countries.

George W. Bush has kept a low profile since leaving the White House a year ago, working on a book about his time in office and developing his presidential library in Dallas.

White House spokesman Robert Gibbs dismissed a question as to why Obama would turn to Bush after criticizing him for the US response to Hurricane Katrina in 2005, saying bipartisan unity was needed.

Aid was beginning to arrive in Haiti, shattered by Tuesday's quake. The Haitian Red Cross said it believed 45,000 to 50,000 people had died and 3 million more were hurt or left homeless.

'To the people of Haiti, we say clearly and with conviction, you will not be forsaken. You will not be forgotten. In this, your hour of greatest need, America stands with you. The world stands with you,' Obama said.

It marks the latest example of the long US history with Haiti. In 1994, then-President Clinton sent a delegation there that negotiated the departure of Haitian military leaders and headed off an American invasion.

Working against time to save as many lives as possible and help Haitians dig out of the rubble, the United States is sending some elements of its armed forces to the impoverished island nation.

Obama said 'one of the largest relief efforts in our recent history' is in motion. Several Coast Guard cutters are already there providing basic services like clean water and technical support for a massive logistical operation.

Up to 3,500 soldiers from the US Army's 82nd Airborne Division will be deployed in Haiti by Sunday, officials said. An advance group of about 125 troops was due to arrive on Thursday and 800 more on Friday.

Another 2,200 Marines from the 22nd Marine Expeditionary Unit at Camp Lejeune, North Carolina, were due to arrive in the coming days for an expected 90-day deployment.

The aircraft carrier USS Carl Vinson, with a crew of 4,000 to 5,000 sailors, was due to arrive by Friday, with 19 helicopters on board. It has three operating rooms, several dozen hospital beds and can produce fresh water.

A US hospital ship, the USNS Comfort, will not arrive until around January 22, with its 12 operating rooms and 250 hospital beds, officials said.

Obama was joined when he made his statement by key leaders of his administration. He was clearly working to avoid any sort of comparison with Bush, who was damaged by his administration's slow initial response to Hurricane Katrina.

Obama, who had been criticized for waiting a few days to speak publicly in response to the Christmas Day attempted bombing of a Detroit-bound plane, said he had made it clear to the officials that Haiti 'must be a top priority for their departments and agencies right now.'

'This is one of those moments that calls out for American leadership,' he said.-Reuters