WASHINGTON - President Barack Obama pledged $100 million for Haiti relief aid on Thursday and enlisted the help of two former U.S. presidents, promising Haitians: You will not be forsaken.

Eager to stay on top of the rapidly unfolding humanitarian crisis, Obama told his top aides that responding to the earthquake aftermath should be their top priority.

Former President Bill Clinton, who is already a United Nations special envoy for Haiti, and former President George W. Bush agreed to a request from Obama to help the quake relief effort. Obama talked by telephone with Bush on Wednesday night about helping out.

Both of them have agreed to take part in this, said White House spokesman Robert Gibbs, promising more details in the next few days.

Clinton and Bush's father, former President George H.W. Bush, joined in a huge international relief effort to help the recovery from the 2004 tsunami that swept South Asia and killed 226,000 in 13 countries after an earthquake in Indonesia.

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

Aid was arriving in a trickle to 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 U.S. 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.

FREQUENT UPDATES

Several Coast Guard cutters are already there providing basic services like water and technical support for a massive logistical operation.

Obama said elements of the U.S. Army's 82nd Airborne Division were arriving on Thursday. A Marine expeditionary unit was also being deployed, as well as the aircraft carrier USS Carl Vinson and a U.S. Navy hospital ship, the Comfort.

In a show of support for Haiti, Obama was joined when he made his statement by key leaders of his administration, including Defense Secretary Robert Gates, Secretary of State Hillary Clinton, Homeland Security Secretary Janet Napolitano and Vice President Joe Biden.

Obama was clearly working to avoid any sort of comparison with Bush, who was damaged by his administration's slow initial response to a domestic disaster, Hurricane Katrina in 2005.

Aides provided frequent updates on what the president was doing on Haiti, such as holding meetings with aides and talking to foreign leaders.

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.

Biden and his wife, Jill, are to be in South Florida to meet with members of the Haitian-American community there about the U.S. response to the crisis.

(Additional reporting by Matt Spetalnick and Jeff Mason; editing by David Storey and Will Dunham)

Barack obama Joe biden George W. Bush