Washington: US administration had in a secret memo authorized targeting of the radical cleric and prominent al-Qaeda's Arab Peninsula arm leader Anwar al-Awlaki, who fell to a drone strike in Yemen.

The killing of Awlaki, a US citizen was authorized by the US justice department in a written opinion, a newspaper reported quoting a former senior intelligence officer.

The officer told the paper that CIA would not have killed the dreaded al-Qaeda leader without authorization, which placed the cleric on the agency's "kill or capture list".

The memo was circulated after senior lawyer from Obama administration reviewed the legal concerns about targeting the US nationals, at home or abroad, said the newspaper.

"What constitutes due process in this case is a due process in war," said one of the officials, who spoke on the condition of anonymity to discuss closely held deliberations within the administration.

The administration has faced a legal challenge and public criticism for targeting Awlaki, who was born in New Mexico, because of constitutional protections afforded US citizens.

The memorandum may represent an attempt to resolve, at least internally, a legal debate over whether a president can order the killing of US citizens overseas as a counter-terrorism measure.

A second American killed in Friday's attack was Samir Khan, a driving force behind Inspire, a magazine produced by al-Qaeda in the Arabian Peninsula. An administration official said the CIA did not know Khan was with Awlaki, but they also considered Khan a belligerent whose presence near the target would not have stopped the attack.

Last year, the Obama administration invoked the state secrets privilege to argue successfully for the dismissal of a lawsuit brought in US District Court in Washington by Awlaki's father, Nasser, seeking to block the targeting of his son.

Judge John Bates found that in Awlaki's case, targeting was a "political question" to be decided by the executive branch.

The decision to place Awlaki on a capture or kill list was made in early 2010, after intelligence officials concluded that he played a direct role in the plot to blow up a jet over Detroit and had become an operational figure within al-Qaeda's affiliate in Yemen.

The newspaper said the CIA reviews every six months to ensure that those targeted for possible killing remained threats as defined by law and presidential findings.