WASHINGTON — The State Department envoy who negotiates detainee transfers from the military prison at Guantánamo Bay, Cuba, is resigning, dealing 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 resignation of Cliff Sloan, a close confidant of Secretary of State John Kerry, 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.

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.”