The Obama administration’s effort to return the largest group of Guantánamo Bay detainees to Yemen, their home country, has stalled, creating a major new hurdle for the president’s plan to close the prison camp in Cuba by next January, American and Yemeni officials say.

“We’re at a complete impasse,” said one American official who is involved in the issue but was speaking without authorization and so requested anonymity. “I don’t know that there’s a viable Plan B.”

The Yemeni government has asked Washington to return its detainees and has said that it would need substantial aid to rehabilitate the men. But the Obama administration is increasingly skeptical of Yemen’s ability to provide adequate rehabilitation and security to supervise returned prisoners. In addition, American officials are wary of sending detainees to Yemen because of growing indications of activity by Al Qaeda there.

The developments are significant for the Obama administration because the 97 Yemeni detainees make up more than 40 percent of the remaining 241 prisoners at Guantánamo Bay. The question of what to do with them “is integral to the process of closing Guantánamo,” said Ken Gude, an associate director at the Center for American Progress who has written about closing the prison camp.