Mr Gates said their use had been authorised by President Barack Obama and would give "precision capability" to the military operation.

Unmanned US drones are already used to target militants along the Pakistan-Afghanistan border.

Libyan rebels have been battling Col Gaddafi's troops since February but have recently made little headway.

"President Obama has said that where we have some unique capabilities, he is willing to use those," Mr Gates told a news conference.

He said two Predators were being made available to Nato as needed, and marked a "modest contribution" to the military operations.

Mr Gates denied that the drone deployment was evidence of "mission creep" in Libya and said there were still no plans to put US "boots on the ground" in Libya.

Armed US Predator drones have been used to deadly effect against militants in Pakistan's tribal areas.

Now they will be employed in similar ways in Libya, where the Pentagon says they will be able to spot Col Gaddafi's forces if they are hiding in civilian vehicles or dug in to residential areas.