"This is an extrajudicial killing, said Kebriaei. Outside a warzone, "the U.S. cannot kill an individual unless that person presents an imminent threat of deadly harm, and lethal force is the last result. That is the standard."
Republican presidential candidate Rep. Ron Paul of Texas also criticized Obama, saying Awlaki should have been tried in a U.S. court like domestic terrorist Timothy McVeigh, who blew up a federal office building in Oklahoma City in 1995.
"If the American people accept this blindly and casually, that we now have an accepted practice of the president assassinating people who he thinks are bad guys, I think it's sad," Paul told reporters in Manchester, N.H.
The drone strike signaled what may be a new phase of U.S. military and intelligence operations in Yemen.
U.S. officials fear that Yemen, which has endured months of deadly violence and tribal fighting, would allow Al Qaeda to strengthen its hold on strategic territory at the intersection of the Middle East and the Horn of Africa.
Awlaki's death may improve Yemen President Ali Abdullah Saleh's standing as a U.S. ally and help him gain international support to hang onto power, analysts said.
Saleh, who survived an assassination attempt in June, unexpectedly returned from Saudi Arabia this month but has been unable to quell a rebellion against him. His military also has battled Al Qaeda militants, who have stormed police stations and government buildings in some towns.
The CIA has increased operations in Yemen in recent months, joining a U.S. military effort that began several years ago. Officials said U.S. special operation troops have worked closely with the Yemeni government in its efforts to crush the Al Qaeda militants.
Awalki, 40, was born in New Mexico when his father, a Yemeni, was studying agriculture at the state university there. He returned with his family to Yemen when he was 7, but returned as a 19-year-old college student. He received a degree in civil engineering at Colorado State University, where he also was head of the Muslim student association.
Though not formally trained as a Muslim scholar, after college he became an imam at mosques in Denver, San Diego and Virginia, and gradually adopted a radical interpretation of Islam and espoused increasing hostility for his adopted home. Before the terror attacks of Sept. 11, 2001, he was in contact with at least two of the hijackers.
Though initially critical of the attacks on the Pentagon and the World Trade Center, he also came under FBI scrutiny and eventually moved back to Yemen, where he was imprisoned by the Yemeni government for two years. Through long sermons posted over the Internet, on CDs and on a web site, Awlaki began denouncing the United States and aligning himself with Al Qaeda.
He became a charismatic voice and clever recruiter but was not a top military commander in the secretive Al Qaeda affiliate battling government forces to create an Islamic state inside Yemen, experts said.
"Killing him is not a big loss inside Yemen," said Saeed Ali Obaid Jamhi, an expert on Islamic militants in the region. "He was not so much involved in the Yemen struggle. He was more of an international figure. He was a spiritual inspiration for jihadis and his death will increase the hatred against the Yemen government for allowing U.S. planes and drones to target people inside Yemen."
In 2010, Awlaki's father, Nasser Awlaki, challenged the legality of the U.S. government issuing an order to kill an American citizen.
A federal judge dismissed the lawsuit last December on the grounds that Alwaki's father, a professor of agriculture in Yemen, could not bring the case on his son's behalf. U.S. District Court Judge John D. Bates ruled that Anwar Alawki would have to file the suit himself.