The US official responsible for negotiating the transfer of detainees from the American-run prison at Guantanamo Bay, Cuba, is leaving his post, the State Department said late Monday.

The US official responsible for negotiating the transfer of detainees from the American-run prison at Guantanamo Bay, Cuba, is leaving his post, the State Department said late Monday.

US Secretary of State John Kerry said Cliff Sloan, Washington's special envoy for Guantanamo closure, resigned in keeping with his agreement to take on the difficult position for just 18 months.  

However, the New York Times, citing an administration official, reported late Monday that Sloan was dissatisfied that too few inmates had been cleared for release by the Pentagon.

Kerry in a statement praised Sloan's "character and commitment."

"Cliff was very skillful negotiating with our foreign partners and allies, and it's a big part of why we moved thirty-four detainees on his watch, with more on the way," the top US diplomat said.

Sloan also played a major role in efforts by the administration to reform the congressional restrictions on foreign transfers.

Sloan's departure comes as the administration strives to empty the detention facility -- one of the very first goals set by Obama when he took office in January 2009.

Inmate transfers to Afghanistan late last week left 132 detainees at Guantanamo.