Thirty-eight US billionaires have pledged at 50% or more of their wealth to charity through a campaign started by investor Warren Buffet and Microsoft founder Bill Gates.

New York Mayr Michael Bloomberg was on the list, as well as CNN founder Ted Turner and entertainment executive Barry Diller.

“The Giving Pledge” has a list of all of the families and individuals who have committed to the project.

The site says that the pledge is a “moral commitment” not a “legal contract”.

The campaign was started in June to convince US billionares to give away at least half of their fortunes either during their lifetimes or after their deaths.

"We've really just started but already we've had a terrific response," Mr Buffett said in a statement.

He added, "The Giving Pledge is about asking wealthy families to have important conversations about their wealth and how it will be used."

Those who pledge their money to "philanthropic causes and charitable organizations" must state publicly their intention through a letter of explanation.

Other billionaires who have promised large sums of money include film maker George Lucas, philanthropist David Rockefeller and oil investor T Boone Pickens.

"I am dedicating the majority of my wealth to improving education. It is the key to the survival of the human race," Mr Lucas wrote in his Giving Pledge letter.

The organization says that a lot of the donors have committed to donating sums far greater than 50% minimum level.

"While the Giving Pledge is specifically focused on billionaires, the idea takes its inspiration from efforts in the past and at present that encourage and recognize givers of all financial means and backgrounds," says Givingpledge.org.

Mr Buffet, along with Mr Gates and his wife, Melinda, held several dinners with the US billionaires in the past year to promote the campaign and urge Americas's financial elite to pledge.

"I've long stated that I enjoy making money, and I enjoy giving it away. I like making money more, but giving it away is a close second," Mr Pickens wrote in a letter on Givingpledge.org.

Mr Buffet, who is the chief executive of the investment firm Berkshire Hathaway, pledge 99% of his money to the Bill & Melinda Gates Foundation and family charities in 2006.

Forbes Magazine estimated that Mr Buffet was worth about $47 billion in March.

Bill Gates, who is rated by Forbes as the world's second wealthiest person, has given away more than $28 billion to his foundation.

There are 403 billionaires living in the US, according to Forbes.