President Barack Obama said Thursday he is uneasy about his own proposal to indefinitely imprison some of the most dangerous terror suspects being held now at Guantanamo Bay. He called it "one of the biggest challenges of my administration."

The president stopped short of abandoning his tentative idea of continuing to hold a small number of detainees in other prisons after Guantanamo closes, which is expected to happen early next year.

But Obama said he has strong reservations about detaining people without bringing them to trial -- a legal quagmire that dogged former President George W. Bush.

"It gives me huge pause," Obama said in an interview with The Associated Press. "And that's why we're going to proceed very carefully on this front. And it may turn out that after looking at all the dimensions of this that I don't feel comfortable with the proposals that surface on how to deal with this issue."

Most of the 229 terror suspects and foreign fighters currently at Guantanamo have been held for up to seven years without being charged. Many will be prosecuted.

One detainee -- suspected U.S. embassy bomber Ahmed Ghailani -- already has been transferred to New York to face federal trial there, starting Sept. 13, 2010. Others are expected to be tried in a military court system created during the Bush administration, which Obama is altering to give the detainees more legal rights.

But government lawyers in both presidents' administrations agree that an unspecified number of detainees should continue to be held because of classified or thin evidence against them. Because such evidence cannot be used at trial, the government fears these most dangerous detainees would be released should they be given their day in court.

"How we deal with those situations is going to be one of the biggest challenges of my administration," Obama said.

If he goes ahead with indefinite detentions, Obama said he would ask Congress to approve it by law -- and not do it himself through an executive order, as some administration officials have privately suggested. It is unclear where such detainees would be kept, but many in Congress oppose the detainees being brought into the U.S.

"It is very important that the American people and Congress, in conjunction with my administration, come up with a structure that is not only legitimate in the eyes of our constitutional traditions, but also in the eyes of the international community," he said.

Constitutional scholars and civil liberties groups have pummeled Obama for considering indefinite detention -- what they called a mirror image of one of Bush's most controversial policies.

On Thursday, Devon Chaffee, attorney for Human Rights First, cautiously praised Obama's hesitation on the issue, calling Obama "correct that one of the greatest assets of the United States in the struggle against terrorism is its adherence to U.S. constitutional values."

Senate Republican leader Mitch McConnell, who has led the growing congressional opposition to closing Guantanamo until Obama details where all the detainees would go, said the prison should remain open for now.

McConnell said in a statement: "On the question of Guantanamo, it became increasingly clear over time that the administration announced its plan to close the facility before it actually had a plan."