WASHINGTON—The Obama administration on Monday lifted its freeze on new military trials at Guantanamo Bay and for the first time laid out its legal strategy to indefinitely detain prisoners who the government says can't be tried but are too dangerous to be freed.

With the policy shifts, Mr. Obama is acknowledging the difficulty he has faced in trying to close the prison at the Guantanamo Bay Naval Base in Cuba, which he had ordered on his second day in office. He is also responding to a congressional ban of moving detainees to the U.S. for trial or detention, which undercut administration plans for civilian trials for some now held at Guantanamo.

Mr. Obama has tried for two years to set policies on terrorism detainees distinct from those of his predecessor, George W. Bush, who established military tribunals, in part, as a way to prevent the release of classified information. Mr. Bush also ordered prisoners held indefinitely as enemy combatants until they no longer posed a threat to the U.S.

Administration officials say the executive order Mr. Obama issued on Monday adds due-process rights absent under Mr. Bush. It calls for periodic reviews of detainees who the administration has determined should be detained indefinitely. The administration has said there are now 48 such detainees who can't be put on trial because, among other reasons, evidence might be tainted by their treatment during questioning and might be deemed by judges to have been coerced.

Administration officials said the reviews would answer constitutional concerns that people held by the government have the right to contest their detention.