The director of the Joint Chiefs of Staff told House lawmakers last week that the U.S. military will not transfer Guantanamo Bay detainees to stateside prisons because current law forbids it.

The statement made by Lt. Gen. William Mayville Jr. in a letter to Congress, first reported by Bloombergappears to create a challenge for President Obama, who is accelerating his push to close the Guantanamo Bay military prison despite objections from lawmakers.

Obama is expected to send his plan to close Guantanamo to Congress soon. The administration intends to move detainees deemed too dangerous for release to stateside prisons. However, Congress has blocked the transfer of Guantanamo detainees to the United States since 2010, and Mayville’s letter made clear that the military will not move detainees to U.S. facilities unless current law barring such transfers is altered.

“Current law prohibits the use of funds to ‘transfer, release or assist in the transfer or release’ of detainees of Guantanamo Bay to or within the United States, and prohibits the construction, modification or acquisition of any facility within the United States to house any Guantanamo detainee,” Mayville wrote to a number of congressmen who served in the military.

“The Joint Staff will not take any action contrary to those restrictions.”

Mayville did say that he supports Obama’s policy to shutter the prison, calling it a “responsible end to holding detainees at Guantanamo.”

Obama has indicated that he may turn to executive action to close the prison if Congress blocks his plan though many have argued that he does not have the legal authority to do so.

“Rumors are that he will act again in an unconstitutional manner,” Sen. John McCain (R., Ariz.) told Politico in November of the possibility that Obama would go around Congress to close Guantanamo. “I think it’s unconstitutional. I think we’d have to go to court.”