The Senate is currently debating an annual defense policy bill that would permit closure of the prison, but only if the president first submits a plan that is approved by the Republican-led Congress.

The U.S. military prison at Guantanamo Bay Naval base on Cuba was set up after the Sept. 11, 2001, attacks for al Qaeda and Taliban detainees. Obama vowed to close it within a year when he came to office in 2009.

Republican lawmakers concerned about housing detainees in the United States have thwarted those moves.

Republican Senator John McCain, head of the Senate Armed Services Committee, has long urged Obama to submit a plan for closing Guantanamo and is backing the language in the policy bill, the National Defense Authorization Act.

McCain says the closure proposal would have to include a plan for hearing the cases of the remaining prisoners at the camp. It also would need a proposal for future detention of the remaining prisoners.

“I’m working with the White House to prepare a plan which we’ll then submit, per longstanding request, to the Congress and discuss with the Congress,” Carter told reporters on his plane to Washington after a trip to Singapore, India and Germany.

“My own view is this is a very constructive step,” he added.

“It’s important to see if we can find a way forward for this that is widely shared enough that we can actually get it done.”

The facility, which once held nearly 800 detainees, now has about 120 inmates after the majority were transferred to their own country or to another state.

It still provokes debates among rights advocates and security officials over whether terrorism suspects should be prosecuted in the U.S. civil legal system or treated as enemy fighters and held under laws of war.