The American official said he and the Pentagon backed the plan proposed by then Secretary of State Hillary Clinton and CIA Director David Petraeus to arm and train militants in Syria.
The plan, which was proposed by the two top US officials in the summer of 2012, called for dispatching weapons to militants in Syria with the help of some regional states.
US President Barack Obama has nominated former Senator Chuck Hagel to replace Panetta as the next Pentagon chief.
Syria has been the scene of unrest since March 2011, and many people, including large numbers of security forces, have lost their lives in the violence.
The Syrian government has repeatedly criticized the US and its regional allies, including Saudi Arabia, Qatar, and Turkey, for funding and arming anti-Damascus militants.
The US has already provided some USD 25 million to armed groups in Syria, calling it non-lethal aid to help them purchase communication devices.
Outgoing US Secretary of Defense Leon Panetta says he supports sending weapons to the militants fighting against the government of Syrian President Bashar al-Assad.
Panetta voiced his support in a testimony to the Senate Armed Services Committee, which centered on the attack on the US consulate in the Libyan city of Benghazi in September 2012.