(Reuters) - The United States has been secretly releasing detainees from a military prison in Afghanistan as part of negotiations with insurgent groups, the Washington Post reported in its Monday editions.

The "strategic release" program has allowed American officials over the past several years to use prisoners as bargaining chips to reduce violence in restive provinces, it said, citing U.S. officials who it said spoke on condition of anonymity.

The freed detainees are often fighters who would not be released under the legal system for military prisoners in Afghanistan. They must promise to give up violence, the report said.

Officials would not say whether those who have been released have later returned to attack U.S. and Afghan troops, the Post said.

Releases have come amid efforts to end the war through negotiation, which is central to the Obama administration´s strategy for exiting Afghanistan, the report said.

Those efforts have yielded little to no progress in recent years. In part, they have been stymied by the unwillingness of the United States to release five prisoners from Guantanamo Bay — a gesture insurgent leaders have said they see as a precondition for peace talks, the report said.

Unlike at Guantanamo, releasing prisoners from the Parwan detention center does not require congressional approval and can be done secretly, the Post said.

The program´s goal is to quell violence in areas where NATO is unable to ensure security. Releases are intended to produce tactical gains, the Post said.

U.S. officials would not say how many detainees have been released under the program, though they said such cases are relatively rare. The program has existed for several years.

"The Afghans have come to us with information that might strengthen the reconciliation process," the newspaper quoted U.S. Ambassador Ryan C. Crocker as saying. "Many times we do act on it."

Releases through the secret program from Parwan must be approved by the top U.S. military commander and military lawyer, and are the only exceptions to the prison´s judicial review board, the Post said.

It quoted one official as saying the procedure was "outside of our normal protocol," the paper said. (© Thomson Reuters 2012. 05/07/12)