Obama administration has taken the extraordinary step of authorizing the targeted killing of an American citizen, the radical Muslim cleric Anwar al-Awlaki, who is believed to have shifted from encouraging attacks on the United States to directly participating in them, intelligence and counterterrorism officials said Tuesday.

 Mr. Awlaki, who was born in New Mexico and spent years in the United States as an imam, is in hiding in Yemen. He has been the focus of intense scrutiny since he was linked to Maj. Nidal Malik Hasan, the Army psychiatrist accused of killing 13 people at Fort Hood, Tex., in November, and then to Umar Farouk Abdulmutallab, the Nigerian man charged with trying to blow up a Detroit-bound airliner on Dec. 25.

American counterterrorism officials say Mr. Awlaki is an operative of Al Qaeda in the Arabian Peninsula, the affiliate of the terror network in Yemen and Saudi Arabia. They say they believe that he has become a recruiter for the terrorist network, feeding prospects into plots aimed at the United States and at Americans abroad, the officials said.

It is extremely rare, if not unprecedented, for an American to be approved for targeted killing, officials said. A former senior legal official in the administration of George W. Bush said he did not know of any American who was approved for targeted killing under the former president. n civil liberties above and beyond anything Dick Cheney dreamt up.