WASHINGTON -- Stymied by Congress so far, the White House is considering issuing an executive order to indefinitely imprison a small number of Guantanamo Bay detainees considered too dangerous to prosecute or release, two administration officials said Friday.

No final decisions have been made about the order, which would be the third major mandate by President Barack Obama to deal with how the United States treats and prosecutes terror detainees.

One of the officials said the order, if issued, would not take effect until after the Oct. 1 start of the upcoming 2010 fiscal year. Already, Congress has blocked the administration from spending any money this year to imprison the detainees in the United States.

The administration also is considering asking Congress to pass new laws that would allow the indefinite detentions, the official said.

Both of the officials spoke Friday on condition of anonymity because they were not authorized to discuss the still-tentative issue publicly. The possibility of an executive order was first reported by ProPublica and The Washington Post.

"A number of options are being considered," said one of the officials.

Asked if the detainees would be indefinitely held overseas or in the United States, the official said: "There's not really a lot of options overseas."

Christopher Anders, senior legislative counsel for the American Civil Liberties Union Washington office, says the organization strongly opposes any plans for indefinite detention of prisoners.

"We're saying it shouldn't be done at all," he said Friday.

The Senate Armed Services Committee just completed work on its FY-2010 defense authorization bill. It was silent on the matter of indefinite detentions, according to a Senate aide familiar with the bill.

Without legislative backing, an executive order is the only route Obama has to get the needed authority.

The order also would only apply to current detainees at Guantanamo -- and not ones caught and held in future counterinsurgent battles.

There are 229 detainees currently being held at Guantanamo. Several have been transferred to United States for prosecution, while others have been sent to foreign nations. The Obama administration is trying to relocate as many as 100 Yemeni detainees to Saudi Arabia for rehabilitation.

Obama said last month he was looking at continued imprisonment for a small number of Guantanamo detainees whom he described as too dangerous to release. He called it "the toughest issue we will face."

"I am not going to release individuals who endanger the American people," Obama said during a May 21 speech at the National Archives. "Al-Qaida terrorists and their affiliates are at war with the United States, and those that we capture -- like other prisoners of war -- must be prevented from attacking us again."

It's not clear how many detainees could fall into that category. Defense and Justice Department officials have privately said at least some could be freed at trial because prosecutors would be reluctant to expose classified evidence against the detainees.

A Pentagon task force is currently reviewing every case to see which are eligible for transfer or release, which could face trial in civilian U.S. courts, which are best suited to some version of a military commission, and which fall into the category to which Obama was referring: too dangerous to free, but against whom legal cases would have a difficult time being mounted because of the manner in which evidence was gathered --coercion by cruel, inhuman or degrading treatment.