The U.S. State Department envoy responsible for negotiating the transfers of terror suspects from the American-run prison at Guantánamo Bay is resigning, officials said Monday evening.

The New York Times described the departure of Cliff Sloan, a close confidant of Secretary of State John Kerry, as "another blow to President Obama’s efforts to close a facility that top administration officials say is a blight on the country’s international standing."

The Times reported that Sloan's resignation, which follows last week's transfer of four men from the prison in Cuba to Afghanistan, "comes as officials at the State Department and the White House have increasingly expressed frustration with the Defense Department’s slow pace of transferring approved prisoners."

But, the paper continued:

In an interview on Monday, Mr. Sloan denied that he was leaving because he was frustrated by foot-dragging at the Pentagon. He said he had always intended to stay a maximum of 18 months, noting that he was right on schedule.

"At this point, we’re in a position to see a lot of progress," he said. "I’m strongly in favor of moving forward as promptly as we can on the president’s commitment to close the facility."

According to ForeignPolicy.com, which broke the news, "Sloan, a high-powered attorney and former publisher of Slate.com, was widely viewed as one of the most aggressive bureaucrats in Washington in pushing to speed up the closure of the facility. Since the beginning of November, Sloan oversaw the transfer of 23 detainees to foreign countries. That upsurge in transfers has reduced the prisoner population to 132, down from a high of 700, and to its lowest point since the early days of the facility’s opening in January 2002."

In a statement, Kerry praised Sloan's work and said he'd "like to have about a hundred Cliff Sloans. He’s the real deal."

An administration official told Politico that no replacement has been named at the moment, but the White House and the State Department are working to find one.