WASHINGTON - (AP) - President Barack Obama declared the killing ofa fiery American-born cleric in Yemen a "major blow" toal-Qaida's most dangerous affiliate, and vowed a vigorous U.S.campaign to prevent the terror network and its partners fromfinding a haven anywhere in the world.

Anwar al-Awlaki, and a second American, Samir Khan, were killedby a joint CIA-U.S. military air strike on their convoy in Yemenearly Friday, U.S. officials said, speaking on condition ofanonymity to discuss classified matters. Both men played key rolesin inspiring attacks against the U.S., and their killings are adevastating double blow to al-Qaida's most dangerous franchise.

Seeking to justify the targeted killing of a U.S. citizen, Obamacalled al-Awlaki "the leader of external operations for Al-Qaidain the Arabian Peninsula," and outlined al-Awlaki's involvement inplanning and directing attempts to murder Americans.

It was the first time the U.S. has described al-Awlaki that way,and Obama appeared to be presenting a legal justification foreliminating him.

"He directed the failed attempt to blow up an airplane onChristmas Day in 2009. He directed the failed attempt to blow upU.S. cargo planes in 2010," Obama said. "And he repeatedly calledon individuals in the United States and around the globe to killinnocent men, women and children to advance a murderous agenda."

Al-Awlaki was a U.S. citizen, born in New Mexico to Yemeniparents, who had not been charged with any crime. Civil libertiesgroups have questioned the government's authority to kill anAmerican without trial.

The White House refused to offer evidence of al-Awlaki's role interrorism or answer questions about the standard for killing anAmerican. Press secretary Jay Carney said any such questions dealtwith the circumstances of the killing and he refused to discussthat.

VIDEO: Security consultant weighs in on death of Anwar al-Awlaki