WASHINGTON- Pentagon leaders knew of the September 11 attack on the US Consulate in Benghazi an hour after it began, but were unable to mobilize reinforcements based in Europe in time to prevent the death of the US ambassador, according to a timeline released on Friday.

Senior defense officials, speaking on condition of anonymity, rejected criticism accusing the Pentagon of failing to move quickly to send reinforcements to relieve the consulate or using armed aerial drones to fire on the attackers.

"The Department of Defense acted quickly after learning of the incidents unfolding in Benghazi," said one official, adding that Marines, special forces and other military assets had either been employed or put in motion during the attack.

"Unfortunately, no alternative or additional aircraft options were available within a timeline to be effective," the official added.

The Obama administration's response to the attack became a highly charged political issue in the last weeks of the presidential campaign. The CIA, which had a base near the consulate, and the State Department have released timelines on the incident.

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