WASHINGTON — President Barack Obama said Thursday that "one of the largest relief efforts in our recent history" is moving toward Haiti as he continued to mobilize the U.S. response to the island's devastating earthquake.

Obama said the U.S. government is initially directing $100 million toward the relief effort, a figure he said would certainly grow over the year. "This is one of those moments that calls out for American leadership," he said.

The first U.S. Army troops — a little more than 100 soldiers from the 82nd Airborne Division — are heading to Haiti, due to leave later Thursday. The troops will find locations to set up tents and other essentials in preparation for the arrival of another roughly 800 personnel from the division on Friday.

They come on top of some 2,200 Marines, also to be sent, as the military ramps up what Secretary of State Hillary Rodham Clinton called a major effort to provide earthquake relief in the form of security, search and rescue, and the delivery of humanitarian supplies. Obama said more than a half dozen U.S. military ships also are expected to help, with the largest, the aircraft carrier USS Carl Vinson, arriving Thursday, and the Navy hospital ship USNS Comfort also deployed.

But, aware that mishandling disaster can quickly become a liability for a president, he warned pre-emptively that it will take hours "and in many cases days" to get the full U.S. relief contingent on the ground, because of the badly damaged roads, airport, port and communications.

"None of this will seem quick enough if you have a loved one who's trapped, if you're sleeping on the streets, if you can't feed your children," Obama said. "So today, you must know that help is arriving. Much, much more help is on the way."

As a start, the president said the U.S. military has secured the severely damaged airport in Port-au-Prince, preparing it to receive round-the-clock deliveries of heavy equipment and emergency supplies being flown in from the United States and countries around the world.

Sensitive to questions about whether the U.S. would need — or choose — to essentially take over Haiti's now almost-nonexistent civil and governmental structure, State Department spokesman P.J. Crowley stressed that U.S. troops sent to Haiti will be under U.S. command but there to augment and support the United Nations mission.

"We're not taking over Haiti," Crowley said. "We are helping to stabilize Haiti, we're helping to provide them lifesaving support and materiel and we're going to be there over the long term to help Haiti rebuild. But, the key is: we are maintaining constant contact with the Haitian government even given the difficult situation. What we're doing is following the priorities that the Haitian government has outlined for us."

Appearing on television network news shows after cutting short an overseas trip, Clinton said it is still too early to make a firm estimate of the number of deaths. But she said officials know that approximately 3 million people have been affected and that "tens of thousands, we fear, are dead."

Clinton noted that the small Caribbean country was still recovering from the damage wrought by last year's storms, with help from Washington, the United Nations and other countries.

She said "this is going to be a long-term effort," from saving lives and providing food, water and medical supplies, to beginning the reconstruction process.

Clinton advised people worried about relatives living in Haiti to call a special information number at the State Department operations center. Obama directed people to go to www.state.gov for information about missing loved ones.

Obama also implored individual Americans to donate to the relief effort, saying they could get information on how to do so through the White House Website at www.whitehouse.gov.

Obama said Americans are being evacuated as quickly as possible. Crowley said 164 Americans have been airlifted out, including 42 non-essential officials and employee family members and 72 private citizens who were taken out on Coast Guard C-130s. Another 50 private citizens left on an Iceland Air flight. There are 360 Americans registered to leave on evacuation flights that will continue Thursday.

"We will not rest until we account for our fellow Americans in harm's way," Obama said.

The president also said he has directed Vice President Joe Biden to travel to South Florida this weekend to meet with members of the Haitian-American community and responders. (AP)