Congressional Republicans launched a pre-emptive strike Monday against a soon-to-be-released plan by President Barack Obama to shutter Guantanamo that would send dozens of detainees to US prisons.

Time is running out for Obama to make good on one of his top national security priorities: closing down the notorious detention facility, a symbol of the controversial counter-terrorism policies of his predecessor George W. Bush.

The administration has spent months working up a plan that would involve the transfer of roughly 50 of the remaining 112 prisoners to US prisons. The others, deemed less dangerous, would be transferred abroad.

Such an act, Republican lawmakers said, would be in "illegal" defiance of Congress, which since 2011 has passed legislation banning the Pentagon from using federal funds to bring the detainees to US soil.

The Pentagon is expected to soon release its plan assessing various US sites, spokesman Navy Captain Jeff Davis said Monday.

"We have a plan that helps achieve the president's goal of closing Guantanamo and we are forwarding that plan to Congress very soon," he said.

"Clearly the Congress's help is needed in doing this."

The plan looks increasingly unlikely to pass muster in the Republican-controlled Congress, raising the prospect of executive action, which would ignite a political firestorm.

At least four sites are under consideration: the US Disciplinary Barracks at Fort Leavenworth military base in Kansas; a federal prison in Florence and a state penitentiary in Canon City, both in Colorado; and a military brig in Charleston, South Carolina.

Republican senators from the states concerned expressed worry on a conference call that arrival of the detainees would present a threat to their communities, and promised a fight in Congress.

Senator Tim Scott of South Carolina said the plan to bring enemy combatants from an isolated location to domestic soil is "mind-boggling," and accused Obama of "gambling national security to keep a campaign promise."

Scott noted that the Charleston brig was two or three minutes from several schools and a dozen churches, not an ideal location to house what he described as "the world's worst terrorists."

Senator Cory Gardner of Colorado pointed out that both potential sites in his state were about an hour from several military facilities, including headquarters of the US Northern Command.

Senator Pat Roberts of Kansas said while it was highly unlikely that extremists would attack Guantanamo to highlight the plight of inmates there, that likelihood would increase if the detainees were transferred to the continental US.

"If you move them to South Carolina or Kansas or Colorado, you immediately paint a bull's eye on those communities," he said.

Roberts said extremists like those in the self-described Islamic State group turn such transfers into a propaganda tool to recruit militants, by attacking communities where their "brothers" are being held.

"They shouldn't be in the United States. We should keep them in Gitmo," he said.