U.S. President Barack Obama, centre, with former presidents Bill Clinton, left, and George W. Bush in the Rose Garden at the White House on Saturday. (Pablo Martinez Monsivais/Associated Press)Bill Clinton and George W. Bush will lead efforts to raise money in their country for relief efforts in Haiti, President Barack Obama said Saturday.

With both former presidents flanking him, Obama praised them as "two gentlemen of extraordinary stature," and said they had agreed to return to help with the largest relief effort in U.S. history.

"Presidents Bush and Clinton will help the American people to do their part," Obama said in a news conference in the White House garden after a meeting in the Oval Office with his two immediate predecessors. "I want to thank both of you for returning to service," Obama said.

"We stand united with the people of Haiti," he said, adding, "that's why the three of us are standing here today."

Hillary Clinton in Haiti

Elsewhere, U.S. Secretary of State Hillary Clinton arrived in Port-au-Prince on Saturday as a part of the U.S. relief effort on the ground in Haiti.

Hillary Clinton is the highest ranking Western official to visit the Haitian capital since the earthquake struck on Tuesday. She was expected to meet with the country's president, Réné Préval.

At the White House, Bush expressed his sympathy for Haitians' suffering and thanked Obama for enlisting his help.

"I am so pleased to answer the call to work alongside president Clinton," he said, adding a blunt message for anyone unsure of how to help Haiti: "Just send your cash."

Bill Clinton said he was looking forward to working with Bush, and pledged a continuing commitment to the Haitian relief effort.

"We will ensure the ongoing integrity of the process. We want to stay with this over the long run," said Clinton, who is also the United Nations special envoy to Haiti.

Obama noted at the press conference that a similar approach enlisting the help of former presidents had been used following the 2004 tsunami that killed about 230,000 people in 14 Asian and African countries.

"This is a model that works," Obama said. "After the terrible tsunami in Asia, president Bush turned to president Clinton and the first president Bush to lead a similar fund."