Obama on Wednesday issued a statement saying the National Defense Authorization Act (NDAA) "includes vital benefits for military personnel and their families, authorities to facilitate ongoing [military] operations around the globe, and important reforms to the military retirement system," as well as other measures.

He added, however, that he is "deeply disappointed that the Congress has again failed to take productive action toward closing the detention facility at Guantanamo." Keeping the prison open "is not consistent with our interests as a Nation and undermines our standing in the world," Obama said.

The White House and the Pentagon are preparing to send to Congress a plan outlining more precisely how they would close the prison and where in the U.S. they might transfer detainees. But the proposal isn’t expected to overcome sizable opposition among Republican lawmakers and some of their Democratic counterparts.

That opposition has put the defense policy bill at the center of the debate over whether Obama would move some detainees from the detention center in Cuba to U.S. facilities without congressional approval. Some legal experts and Obama administration allies argue that the restrictions are unconstitutional, and are urging Obama to move the detainees and close the prison despite the provisions. The White House has said it is focused on working with Congress, but hasn’t ruled out other options.

"As I have said repeatedly, the executive branch must have the flexibility, with regard to the detainees who remain at Guantanamo, to determine when and where to prosecute them, based on the facts and circumstances of each case and our national security interests, and when and where to transfer them consistent with our national security and our humane treatment policy," Obama said in Wednesday's statement.

But after Obama signed the bill, civil rights attorneys accused him of conceding to political opponents.

"The president has nobody but himself to blame for the failure to close Guantanamo," Wells Dixon, an attorney with the Center for Constitutional Rights who has defended several Guantanamo detainees, told Al Jazeera. "I hope on Thanksgiving he stops to think about the people he has stranded in his illegal offshore prison and the families that miss them terribly."

David Remes, a human rights lawyer who also represents numerous Guantanamo detainees, told Al Jazeera that transferring prisoners from one facility to another would still violate their human rights.

“It doesn’t matter that Congress has prevented him from bringing detainees to the U.S., only to hold them indefinitely in detention without charge. That’s just changing the zip code,” Remes said. “What matters is that time after time he let politics stand in the way of sending detainees back to their home countries or resettling them in other countries. That’s the great tragedy, and he has only himself to blame.”

Meanwhile, House Speaker Paul Ryan, R-Wis., said Obama's approval of the NDAA "reaffirms longstanding prohibitions on the transfer of Guantanamo Bay detainees to the United States."

The legislation will guide Pentagon policy for nearly all of the remainder of Obama's tenure. In addition to the detainee ban, it includes a 1.3 percent pay increase for service members, authorization for lethal assistance to forces fighting Russian-backed rebels in Ukraine, and funding to help Iraqi forces fight Islamic State in Iraq and the Levant, or ISIL.

Obama in October vetoed an earlier version of the bill over a dispute — later resolved — about the way defense programs would be financed. He did not repeat the threat over the Guantanamo provisions, mindful that he would not have the votes to sustain a veto. The legislation passed with overwhelming support in both the House and Senate.

Al Jazeera and The Associated Press. Jenifer Fenton contributed to this report.