Defense Department admits that former Guantanamo Bay prisoners are back fighting against the U.S.

Defense Secretary Chuck Hagel on Thursday acknowledged that some of the former prisoners the U.S. had been holding in detention facilities in Guantanamo Bay, Cuba, have returned to the battlefield and are fighting against the United States.

Hagel admitted this after being asked whether he was worried about former prisoners taking up arms against the U.S. in Iraq and Syria.

"Well, we know that some of the detainees that have come out of Guantanamo have gone back to the fight, to the battlefield," he replied. "We're aware of that."

At the same time, Hagel said he still believes it's best for the U.S. to close down Guantanamo Bay and reach agreements to have other countries host prisoners taken in the war against terrorism. Hagel said those commitments are needed because it's an "imperfect" and "dangerous" world.

"This is why we pay so much attention to getting commitments from host countries in securing those commitments and doing everything we can within our power to assure that those commitments, not to allow those detainees to go beyond what is required in order to secure them in these different host countries that take them," he said.

Hagel then acknowledged again, "But we do know that some have joined the fight."

Hagel wasn't more specific, but Fox News reported Thursday night that as many as 30 former detainees have joined the Islamic State and other terrorist groups.

Hagel's admission is likely to make it harder for the Obama administration to argue for the closure of Guantanamo Bay. The administration has wanted to close down the facility for years, and Obama has said it could be done in a way that doesn't put Americans in any danger.

On Thursday, House Armed Services Committee Chairman Buck McKeon (R-Calif.) wrote a letter to Hagel urging him not to release any more detainees.

"The re-engagement rate of former Guantanamo detainees is ever-increasing and there are public reports that detainees are specifically rejoining the fight alongside ISIL," he wrote. "The U.S. government must not release terrorist detainees at the same time we have committed U.S. service members to fight ISIL."

Read McKeon's letter here:

Mckeon