The US has authorised the use of armed, unmanned Predator drones over Libya to give "precision capabilities".
Libyan rebels have been battling Col Gaddafi's troops since February but have recently made little headway.
Adm Mullen also said there was no sign of al-Qaeda in the Libyan opposition.
Speaking to US troops in Iraq, he said radical groups might try to take advantage of the Libyan uprising, but added: "We're watchful of it, mindful of it and I just haven't seen much of it at all. In fact, I've seen no al-Qaeda representation there at all."
Last month, a Nato commander said US intelligence had picked up "flickers" of al-Qaeda activity among the rebels.
Meanwhile Senator John McCain has visited the rebel stronghold of Benghazi, the most senior US official to do so since the conflict began in February.
He called on all countries to recognise the rebels' Transitional National Council as "the legitimate voice of the Libyan people", and to offer weapons and training.