WASHINGTON (Reuters) - President Barack Obama, in a setback to hopes for the quick closing of the Guantanamo Bay prison, reluctantly signed a bill on Friday barring suspects held there from being brought to the United States for trial.

Making plain he would fight to repeal language in the law obstructing civilian U.S. trials for Guantanamo terrorism suspects, Obama said he was left with no choice but to sign the defense authorization act for fiscal 2011.

“Despite my strong objection to these provisions, which my administration has consistently opposed, I have signed this act because of the importance of authorizing appropriations for, among other things, our military activities in 2011,” Obama said in a statement.

Obama has vowed to close the U.S. military prison at Guantanamo Bay, Cuba, which has drawn international condemnation for the treatment of detainees, but has met stiff resistance at home.

The bill includes sections blocking funding for the transfer of suspects from the Guantanamo prison to the United States. It also restricts the use of funds to ship them to other countries, unless specified conditions are met.

“The prosecution of terrorists in federal court is a powerful tool in our efforts to protect the nation and must be among the options available to us,” Obama said. “Any attempt to deprive the executive branch of that tool undermines our nation’s counterterrorism efforts.”

Senate Republican leader Mitch McConnell said the legislation Obama signed into law showed there was overwhelming bipartisan opposition to bringing Guantanamo detainees to the United States for trial and detention.

“When it comes to terrorism, we should err on the side of protecting the American people,” McConnell said in a statement.

The provisions expire on September 30, at the end of the current fiscal year. What happens at that point depends on what Congress decides on defense authorization.

Until then, the law will make it very difficult for the Obama administration to pursue criminal trials for terrorism suspects, including the self-professed mastermind of the September 11 attacks, Khalid Sheikh Mohammed, who had been slated to face a trial in New York.

FIGHT FOR REPEAL

“My administration will work with the Congress to seek repeal of these restrictions, will seek to mitigate their effects, and will oppose any attempt to extend or expand them in the future,” said Obama, who pledged during his 2008 presidential campaign to close Guantanamo.

There are still 174 detainees at the Guantanamo prison and about three dozen were set for prosecution in either U.S. criminal courts or military commissions. Republicans have demanded the trials be held at Guantanamo.

In a May 2009 speech, Obama said there was a need for “prolonged detention” for some terrorism suspects who could not be tried but posed a threat to security.

U.S. officials say trials are not possible in some cases because evidence was obtained through torture or is classified.