Notching another success in his pursuit of top terror leaders, US President Barack Obama said Anwar Al-Awlaqi was the leader of external operations for Al-Qaeda in the Arabian Peninsula (AQAP) and had plotted the murder of innocent Americans.
"The death of Awlaqi is a major blow to Al-Qaeda's most active operational affiliate. (It) marks another significant milestone in the broader effort to defeat Al-Qaeda and its affiliates," Obama said.
He said Awlaqi's killing in an air raid in Yemen was a tribute to the US intelligence community and to Yemen's cooperation with the United States in a common anti-terror campaign.
That appeared to be a hint that there may be truth to reports CIA intelligence and drone aircraft may have been used in the killing, though the White House would not be drawn into details of the operation.
US officials believe Awlaqi played a significant role in the attempt to bring down a US airliner over Detroit by an assailant with explosives sewn into his underwear on December 25, 2009.
He was also believed to have coordinated the thwarted 2010 plot to blow up cargo aircraft bound for the United States and had called for attacks against US and Arab governments across the world.
Obama also warned, during a retirement ceremony for the chairman of the Joint Chiefs of Staff Admiral Mike Mullen, that though "weakened," AQAP was still "dangerous."
The president also appeared to preview an expanded US sweep against extremists, following action against terror suspects in recent months in Yemen, Pakistani tribal regions and Somalia, by Washington and its allies.
"Going forward, we will remain vigilant against any threats to the United States or our allies and partners," Obama said.
"Make no mistake, this (attack) is further proof that Al-Qaeda and its affiliates will find no safe haven anywhere in the world.
"Working with Yemen and our other allies and partners, we will be determined, we will be deliberate, we will be relentless, we will be resolute in our commitment to destroy terrorist networks that aim to kill Americans."
Defense Secretary Leon Panetta billed Awlaqi's death in an airstrike many reports attributed to a US covert anti-terror program as the latest in a string of US success, which included the killing of Osama bin Laden in May.
"It's been a bad year for terrorists. We just have seen another major blow to Al-Qaeda," he said, describing Awlaqi as "someone who was truly an operational arm of Al-Qaeda in this node of Yemen."
"We had always had tremendous concern that after getting Bin Laden, someone like Awlaqi was the primary target for us."
Yemen's defense ministry, which first announced Awlaqi's death, also said that Pakistani-American Samir Khan, who edited Al-Qaeda's English-language magazine "Inspire," was killed in the same air strike.
But the White House declined to confirm reports that US forces or CIA drones were involved in the raid on Awlaqi's convoy, as civil rights groups warned of a legal quagmire over the possible killing of Americans by the US government.
White House spokesman Jay Carney declined to answer repeated questions in his daily briefing about the broader implications of Awlaqi's killing, despite media reports that a US drone aircraft was involved in the operation.
"I'm not going to speak hypothetically. I'm not going to speak about the circumstances of his death," Carney said.
"I'm not going to talk about the circumstances of Awlaqi's death. And I'm not going to acknowledge or concede or accept premises embedded in questions."
Pardiss Kebriaei, an attorney at the Center for Constitutional Rights who worked on the lawsuit, said if Awlaqi were indeed killed by US forces or with their help, it would be illegal.
"If it was in the absence of imminent threat of deadly harm, this was an illegal killing under the US constitution and international law," she told AFP, and called for "an immediate investigation" by an independent party.
A US official however said that in general terms it would be lawful for the United States to target high-level leaders of "enemy forces" regardless of their nationality, under US and international law that recognized the right of self-defense.