The recent security threat emanating from Yemen has complicated President Obama’s latest push to close the military prison at Guantanamo Bay, Cuba, reviving doubts among conservative lawmakers about whether it is safe to return Yemeni detainees to their turbulent home country.

More than half of the 166 prisoners being held in Guantanamo are from Yemen, and the transfer of many of them is essential to Obama’s long-promised goal of closing the detention center.

But the recent threat involving al-Qaeda in the Arabian Peninsula, as the terrorist group’s Yemen-based branch is known, underlines the national security and political obstacles ahead. The United States and other countries this week shut down their embassies in Yemen, citing concerns about security.

“Since it’s now well-known that Yemen-based al-Qaeda is actively plotting against us, I don’t see how the president can honestly say any detainee should be transferred to Yemen,” Sen. Saxby Chambliss (Ga.), the ranking Republican on the Senate Select Committee on Intelligence, said in a statement. “Sending them to countries where al-Qaeda and its affiliates operate and continue to attack our interests is not a solution.”

It is not the first time that Yemen has emerged as the prime obstacle to Obama’s attempt to close the prison, which he has called an important terrorist recruiting tool and a $350 million annual burden on the strained federal budget.

As terror threats and embassy closings come into focus in the Middle East, Nia-Malika Henderson talks to former Ambassador Ryan Crocker about the challenges in the region. (The Washington Post)

After the attempted bombing in 2009 of a Detroit-bound airliner by a young Ni­ger­ian trained in Yemen, Obama suspended the transfer of Yemeni detainees from Guantanamo. The effort to close the prison had slowed ever since.

But a hunger strike by a majority of Guantanamo detainees this year pushed questions about the prison’s legality and Obama’s languishing promise to close it back into the public debate. In a May speech at the National Defense University, the president announced that he was lifting his moratorium.

That has set in motion, albeit slowly, an assessment process led by the Defense Department and known as the Periodic Review Board. The board will hold hearings for about 70 detainees who have not been cleared for return, about 30 of whom are from Yemen.

An additional 86 detainees — the majority of them Yemeni — were cleared more than four years ago for repatriation or resettlement, although the White House must notify Congress before any transfer can occur. Late last month, the administration notified Congress that it would soon repatriate two Algerian detainees.

The Yemeni government is a U.S. ally in counterterrorism operations, tacitly allowing drone strikes against suspected militant targets. But the government’s weak hold on much of the rugged country has allowed al-Qaeda’s most potent franchise to flourish in the hinterlands and in some cities.

Last week, Yemeni President Abed Rabbo Mansour Hadi visited the White House to discuss repatriation efforts, drawing praise from Obama for his help on security issues.

At the same time, the State Department’s new special envoy for Guantanamo closure efforts, Clifford Sloan, is negotiating the terms of prisoner repatriation to Yemen to determine how the government intends to prevent their return to the battlefield.

Those measures could include a short-term detention on return, regular monitoring, or participation in a reintegration program, which has proven successful in Saudi Arabia and Kuwait.

During his meeting with Obama, Hadi said he intended to start “an extremist rehabilitation program,” although the cash-strapped government’s efforts to do so have been hampered by the cost.

“They obviously have a number of good candidates for repatriation, including those cleared by the national security process more than four years ago,” said Christopher Anders, senior legislative counsel at the American Civil Liberties Union. “Nobody sent back to Yemen is just going to be dropped off at the airport.”

The new Yemen threat comes as a divided Congress prepares for a fall debate over rival elements within the defense authorization legislation that could help or hinder Obama’s ability to shutter the prison.

The Senate Armed Services Committee has approved proposals from the chairman, Sen. Carl Levin (D-Mich.), that would give Obama more flexibility in transferring prisoners and allow some to be detained, to be prosecuted or to receive specialized medical care inside the United States. Chambliss and other Republicans on the committee objected in a minority report.

The GOP-controlled House, meanwhile, approved a measure that would block any Yemeni prisoner transfers. The administration and many Hill Democrats objected, and those elements will be the subject of debate when Congress takes up the matter again as early as next month.

“I don’t think that the release of any Yemeni detainees is imminent,” said David Remes, a civil liberties lawyer who represents 14 Yemenis being held at Guantanamo. “I do believe that the heightened security concerns would complicate matters if the president were planning to transfer Yemeni detainees very soon.”

Two of Remes’s Yemeni clients are scheduled to be among the first to go before the Defense Department’s Periodic Review Board, which will determine what security risk each detainee would present if returned.

Remes visited Guantanamo last month to meet with his clients about the board hearing, which is likely before the end of the year.

Lawyers and civil liberties advocates say that the fact that the first two detainees to undergo the review are from Yemen underscores the importance that Obama has placed on repatriating that group. But Remes is worried that the latest security threat may disrupt any planned transfers to Yemen and, perhaps, delay the review process for others.

“The security concerns are unfounded,” he said. “It’s easy to demagogue the issue, and so far President Obama has rolled over every time he has faced controversy on this.”

Senior administration officials say Obama has no plans to reconsider his decision to lift the transfer moratorium, saying that doing so was based not on security assessments but on the improved cooperation and capabilities of Hadi’s 20-month-old government.

“We are continuing to review detainees from Yemen on a case-by-case basis, as we do with detainees from all other countries,” said Caitlin Hayden, a spokeswoman for the National Security Council.

Julie Tate contributed to this report.