The release of four Syrians, a Tunisian and a Palestinian, who arrived in South America aboard a U.S. military transport plane, represented the largest single group to leave the internationally condemned U.S. detention camp since 2009, U.S. officials said.
President Barack Obama took office nearly six years ago promising to shut the prison, citing its damage to America's image around the world. But he has been unable to do so, partly because of obstacles posed by the U.S. Congress.
The transfer to Uruguay had been delayed for months. A move initially planned earlier this year was apparently held up by the Defense Department.
Differences over the pace of such transfers, said one U.S. official, added to friction between Defense Secretary Chuck Hagel and Obama's inner circle, culminating in Hagel's resignation last month.
The release of the six was put off again in August when Uruguay became concerned about domestic political risks in the run-up to its October presidential election. But outgoing President Jose Mujica then pressed ahead with the transfer.
Upon their arrival in Montevideo, they were taken to a hospital for medical examinations, the U.S. official said.
"We are very grateful to Uruguay for this important humanitarian action," said Clifford Sloan, Obama's State Department envoy on Guantanamo, who negotiated the resettlement deal in January. "The support we are receiving from our friends and allies is critical to achieving our shared goal of closing Guantanamo, and this transfer is a major milestone."
Seven other prisoners have been transferred from Guantanamo since early November, including three to Georgia, two to Slovakia, one to Saudi Arabia and one to Kuwait. With Sunday's release, the prisoner population has been whittled down to 136.
The jail 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. Most have been held for a decade or more without being charged or given a trial.
CLEARED FOR TRANSFER
The detainees released on Sunday were cleared for release long ago and are not regarded as security threats. But U.S. authorities did not want to send them home, saying countries such as Syria, where a civil war is raging, were too risky.
Among the Syrians sent to Uruguay was Jihad Diyab, who recently mounted a legal challenge against the U.S. military's force-feeding of hunger strikers at Guantanamo.
The Pentagon identified the other Syrians as Ahmed Adnan Ahjam, Ali Hussain Shaabaan and Omar Mahmoud Faraj. Also released were a Tunisian, Abdul Bin Mohammed Abis Ourgy, and a Palestinian, Mohammed Tahanmatan.
The U.S. official said Uruguay agreed to "security arrangements" and that the six would be "free men". But he declined to say whether they would be allowed to travel abroad.
Mujica, a former leftist guerrilla jailed during a 1973-1985 military dictatorship, has called the prison a "disgrace". He had urged Washington to free three imprisoned Cuban spies as a reciprocal gesture, but U.S. authorities said this was never part of the negotiations for accepting the Guantanamo detainees.
More are expected to be repatriated or sent to countries other than their homelands by year-end, the official said.
Signaling White House frustration with Hagel's handling of Guantanamo transfers, Obama's national security adviser, Susan Rice, took the unusual step of sending him a memo in May urging him to streamline the process, the official said. Ashton Carter, Obama's nominee to replace Hagel, could face similar pressure.
But Obama still faces major obstacles, among the biggest being the Yemeni detainees who make up more than half of the inmate population, with most cleared for transfer but unable to return home due to the chaotic security situation in Yemen.
There are concerns in Washington that some might return to the battlefield. Yemen is the base for an al Qaeda affiliate that was the target on Saturday of a U.S. hostage rescue attempt, which ended with an American journalist and a South African teacher being killed by their captors.