The frontier road parallels the security fence leading to the Northeast gate at Guantanamo Bay U.S. Naval Base, March 8, 2013. REUTERS/Bob Strong

By Matt Spetalnick

WASHINGTON (Reuters) - Four Afghans held for over a decade at the U.S. military prison at Guantanamo Bay, Cuba, have been sent home, the Pentagon said on Saturday, the latest step in a gradual push by the Obama administration to close the jail.

The men were flown to Kabul overnight aboard a U.S. military plane and released to Afghan authorities, the first such transfer of its kind to the war-torn country since 2009.

With the repatriation of the four Afghans, Guantanamo’s detainee population has been whittled down to 132. Several more prisoners of "various nationalities" are expected to be transferred before the end of the year and a further unspecified number in succeeding weeks, according to a senior U.S. official.

Obama promised to shut the internationally condemned prison when he took office nearly six years ago, citing the damage it inflicted on America's image around the world. But he has been unable to do so, partly because of obstacles posed by Congress.

The repatriation of the four Afghans, identified as "low-level detainees" who were cleared for transfer long ago and are not considered security risks in their homeland, had been in the pipeline for months.

But in what one senior U.S. official described as an expression of growing confidence in the new Afghan president, Ashraf Ghani, who took over from Hamid Karzai in September, Washington pressed ahead with the transfer after he formally requested it.

The continued detention of Afghans at Guantanamo -- eight remain there -- has long been deeply unpopular across the ideological spectrum in Afghanistan.

The release comes at a time when most U.S. troops are due to leave Afghanistan by year-end, even as Taliban insurgents are intensifying their bloody campaign to re-establish their hardline Islamist regime that was toppled in a U.S.-backed military intervention in 2001.

All four men – identified as Shawali Khan, Khi Ali Gul, Abdul Ghani and Mohammed Zahir – were originally detained on suspicion of being members of the Taliban or affiliated groups.

But a second U.S. official said: "Most if not all of these accusations have been discarded and each of these individuals at worst could be described as low-level, if even that."

The Afghan government gave the United States "security assurances" for the treatment of the former prisoners and was expected to reunite them with their families, the official said.

CLOSING PRISON WON'T BE EASY

Guantanamo was opened by Obama's predecessor, George W. Bush, after the Sept. 11, 2001, attacks on the United States, to house terrorism suspects rounded up overseas, with Afghans originally the largest group. Most of the detainees have been held for a decade or more without being charged or tried.

Two weeks ago a U.S. Senate report delivered a scathing indictment of the harsh Bush-era interrogation program used on terrorism suspects. Obama banned the techniques when he took office in 2009.

Thirteen other prisoners of various nationalities have been transferred from Guantanamo since early November, including six who were sent to Uruguay for resettlement earlier this month.

But emptying the prison will not be easy.

In a statement issued on Friday, Obama renewed his complaints about restrictions on Guantanamo transfers that Congress kept in place in a recent defense spending bill. "The Guantanamo detention facility's continued operation undermines our national security," he said. "We must close it."

Among the detainees released this weekend, Khan, 51, was sent to Guantanamo 11 years ago "on the flimsiest of allegations", according to the Center for Constitutional Rights. His lawyers said he had been a driver for the Karzai government.

According to a Guantanamo database compiled by the New York Times and National Public Radio, Gul, 51, was arrested in 2002 and accused of being a Taliban intelligence officer. He insisted he never worked for the group and that two of his "enemies" had turned him over to U.S. troops.

Ghani, 42, was captured in 2002 as a suspected member of a Taliban-linked faction and was originally accused of "war crimes". He said someone falsely accused him of carrying out a rocket attack, the documents show, and was cleared by an inter-agency review.

Zahir, 61, was arrested in 2003 and accused of links to Taliban weapons caches, but he denied any connection and was also cleared for transfer.

(Editing by Mark Heinrich and Crispian Balmer)