The Giving Pledge was started in June by Gates, whose $53 billion fortune places him second on the Forbes magazine list of the world's richest people, and Buffett, who ranks third on the list. They wanted to persuade hundreds of U.S. billionaires to give away most of their fortune during their lifetime or after their death and to publicly state their intention with a letter of explanation.

"We've really just started but already we've had a terrific response," Buffett said in a statement. "The Giving Pledge is about asking wealthy families to have important conversations about their wealth and how it will be used.

"We're delighted that so many people are doing just that -- and that so many have decided to not only take this pledge but also to commit to sums far greater than the 50 percent minimum level," he said.

The Giving Pledge does not accept any money but asks billionaires to make a moral commitment to give away their wealth to charity.

Making a difference

"I am enthusiastically taking the Giving Pledge, and nearly all of my net worth will be given away in the years ahead or left to my foundation," Bloomberg wrote in his Giving Pledge letter. "Making a difference in people's lives -- and seeing it with your own eyes -- is perhaps the most satisfying thing you'll ever do."

The billionaires announcing their pledge on Wednesday join real estate and construction billionaire Eli Broad, venture capitalist John Doerr, media entrepreneur Gerry Lenfest and former Cisco Systems Chairman John Morgridge who have already committed to giving away most of their wealth.

Buffett, who made his fortune with insurance and investment company Berkshire Hathaway Inc, Gates and his wife, Melinda, held several dinners with a couple of dozen rich Americans in the past year to urge them to make the pledge.

Buffett pledged in 2006 to give away 99 percent of his wealth to the Bill & Melinda Gates Foundation and family charities. Bill and Melinda Gates have so far donated more than $28 billion of their fortune to their foundation.

Since the foundation began in 1994, it has given away more than $22 billion for health improvements in poor countries and to improve access for Americans to opportunities they need to succeed in school and life.

Buffett said he and Bill Gates also will meet with groups of wealthy people in China and India within the next six months to talk about philanthropy. They hope the idea of generosity will spread, but they have no plans to lead a global campaign, Buffett said.

Gates and Buffett estimate their efforts could generate $600 billion dollars in charitable giving. In 2009, American philanthropies received a total of about $300 billion in donations, according to The Chronicle of Philanthropy.

"I've long stated that I enjoy making money, and I enjoy giving it away," Pickens said in his Giving Pledge letter. "I'm not a big fan of inherited wealth. It generally does more harm than good."

Forbes said the United States is home to 403 billionaires, the most in the world.