NEW YORK (Reuters) - Financier George Soros said on yesterday he would give $100 million to Human Rights Watch to help the US-based group strengthen globally after America's moral authority suffered under the Bush administration.

The billionaire, whose donation to the rights organisation will be made over the next 10 years, said the United States' blemished rights record had lessened its influence as a global rights advocate and urged other countries to fill the void.

"The United States, being the main promoter of human rights, has lost the moral high ground that it used to occupy," Soros, 80, told Reuters in an interview. "Other countries believe in it too so they need to be mobilised and they need to be activated because they can now have a bigger impact."

The United States has been condemned internationally for its handling of terrorism suspects, particularly under former US President George W. Bush. Critics have accused President Barack Obama of softening a US commitment to promote rights and not moving fast enough to reverse Bush-era policies.

The United States joined the UN Human Rights Council last year after boycotting it under the Bush administration.

Soros, a staunch Democratic Party supporter who was among the earliest big-name supporters of Obama's presidential bid, said he believed the US rights record had made it difficult for Human Rights Watch, noting his own Open Society Foundations had suffered problems.

"My foundation is promoting democracy and open society throughout the world ... and we ran into a lot of difficulties during the Bush administration because of the changing attitudes towards the United States," he said.

Soros said his gift to Human Rights Watch was a "challenge grant". The group must also try to raise another $100 million over the next decade, mainly from outside the United States, but the Soros grant is not conditional on that, he said.

He said he became involved with Human Rights Watch in the early 1980s when he began his philanthropic activities by attending the group's weekly meetings. His $100 million grant is the largest Soros has made to a non-governmental organisation.

"It's an organization that I know very well and I can vouch for its efficiency and having kept its spirit," Soros said.

Soros, who is ranked 35th on Forbes list of the world's richest people with an estimated fortune of $14 billion, gives away more than $600 million a year, but has changed his plans to give away all of his wealth before his death.