U.S. 'stuck' with Guantanamo because of 'legal difficulties', claims defence chief

"The brutally frank answer is that we're stuck and we're stuck in several ways," Mr Gates told a U.S. Senate hearing when asked about his desire to shut down the detention site for terrorism suspects at the American naval base in Cuba.

Human rights groups and many governments, including allies of the United States, have called on the Bush administration to close the prison, saying it violates international legal standards and harms America's standing in the world.

Stuck: A U.S. soldier guard a recreation area in Camp 6 at Guantanamo Bay

Mr Gates has said he wanted to close the site, where inmates have been held for years without trial, after he took over from Donald Rumsfeld at the Pentagon in late 2006 and assigned officials to look into the issue.

But the former CIA chief said the effort had run up against several major problems.

The first was that the United States had identified about 70 prisoners who could be returned home in theory but not in practice.

"The problem is that either their home government won't accept them or we're concerned that the home government will let them loose once we return them home," he said.

Some 270 detainees remain in Guantanamo Bay - including Khalid Sheik Mohammed, the alleged architect of the September 11 attacks - and more than 500 have left since the site opened in January 2002, according to the U.S. military.

The Pentagon says some 36 former Guantanamo inmates are "confirmed or suspected of having returned to terrorism." It has released details of 13 men it says are in this category.

Mr Gates cited a Kuwaiti former inmate who carried out a suicide bombing in Mosul in northern Iraq last month. Both the man's family and the U.S. military have said he conducted the attack.

Rights activists questioned Gates' arguments and said the Bush administration lacked the will to close Guantanamo.

"The secretary's comments really are astounding in light of the money, resources and personnel the Department of Defense spent getting America into this mess at Guantanamo," said Vincent Warren, executive director of the Center for Constitutional Rights.

Jennifer Daskal, senior counterterrorism counsel at Human Rights Watch, said other nations would be more willing to accept detainees if they believed Washington was serious about closing the site.

Captive: Khalid Sheikh Mohammed, the alleged September 11 mastermind is beingheld at Guantanamo

In the Senate, Gates said the United States has also failed to come up with a solution for inmates who cannot be freed for security reasons but will not be charged under the military commissions system for trying war crimes suspects.

"We just have a hard time figuring out ... what do you do with that irreducible 70 or 80 or whatever the number is," he told the defense subcommittee of the Senate Appropriations Committee.

There was also a widely held reluctance to house any of the prisoners in the United States, Gates said.

"We have a serious 'not in my backyard' problem. I haven't found anybody who wants these terrorists to be placed in a prison in their home state," Gates said. "Those three problems really have brought us to a standstill."