The Pentagon and the CIA have asked the White House to decide on a more permanent approach for those it is unwilling to set free or turn over to US or foreign courts, the Washington Post said in a report that cited intelligence, defence and diplomatic officials.
Some detentions could potentially last a lifetime, the newspaper added.
But influential senators swiftly denounced the idea on Sunday as probably unconstitutional.
"It's a bad idea. So we ought to get over it and we ought to have have a very careful, constitutional look at this," Republican Senator Richard Lugar of Indiana, chairman of the Senate foreign relations committee, said on Fox News.
Senator Carl Levin of Michigan, senior Democrat on the armed
services committee, also opposed the reported plan.
George Bush says terror suspects
are a threat to US security
"There must be some modicum, some semblance of due process ... if you're going to detain people, whether it's for life or whether it's for years," Levin said, also on Fox.
The White House did not immediately respond to a request for comment. The Department of State also declined comment and a Pentagon spokeswoman had no information on the reported plan.
As part of a solution, the Defence of Department, which holds 500 prisoners at Guantanamo Bay, plans to ask the US Congress for $25 million to build a 200-bed prison to hold detainees who are unlikely to ever go through a military tribunal for lack of evidence, defence officials told the Post.
The new prison, dubbed Camp 6, would allow inmates more comfort and freedom than they have now, and would be designed for prisoners the government believes have no more intelligence to share, the newspaper said.
"It would be modelled on a US prison and would allow socialising among inmates," the daily added.
War on terror
"Since global war on terror is a long-term effort, it makes sense for us to be looking at solutions for long-term problems," Bryan Whitman, a Pentagon spokesman, was quoted as saying.
"There must be some modicum, some semblance of due process ... if you're going to detain people, whether it's for life or whether it's for years"
Senator Carl Levin,
armed services committee
"This has been evolutionary, but we are at a point in time where we have to say: 'How do you deal with them in the long term?'"
The Post said the outcome of a review under way would also affect those expected to be captured in the course of future counterterrorism operations.
One proposal would transfer large numbers of Afghan, Saudi and Yemeni detainees from the US military's Guantanamo Bay, Cuba, detention centre into new US-built prisons in their home countries, it said.
The prisons would be operated by those countries, but the Department of State would ask them to abide by recognised human rights standards and would monitor compliance, a senior administration official was quoted as saying.