Armed US Predator drones are carrying out missions over Libya, Defence Secretary Robert Gates has said.
Mr Gates said their use had been authorised by President Barack Obama and would give “precision capability” to the military operation.
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 armed, unmanned 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.
“There’s no wiggle room in that,” he said.
Gen James Cartwright, vice-chairman of the Joint Chiefs of Staff, said the first mission had taken place on Thursday but the drones had turned back due to bad weather.
He said the drones – which can fly at a lower altitude than conventional fighter jets – were “uniquely suited for urban areas”, providing improved visibility of tanks and other potential targets.
Earlier on Thursday, Libyan rebels seized control of a border post on the Tunisian border after about 100 government soldiers fled, say reports.
The post is on the road between the Libyan town of Nalut and Dehiba in Tunisia.
The move marks a rare advance against government troops in the west of the country and followed intense fighting in the western mountain region.
Restrictions on journalists in remote areas of Libya mean it is hard to independently verify such reports.
Fierce fighting is also continuing in the besieged western city of Misrata, with at least seven people killed on Thursday.
Medics say more than 1,000 people have died in weeks of fighting.
Residents say they are being targeted in the streets by snipers firing indiscriminately.
Rebels in Misrata claim to have found remnants of cluster bombs but the Libyan government has so far denied the charge.
The BBC’s Orla Guerin in Misrata says she has seen the bombs herself and that doctors have told her they are causing increasingly horrific injuries, with some civilians losing limbs.
On Wednesday, two journalists died in a mortar attack in the city – Tim Hetherington, a British-American filmmaker and Chris Hondros, an American photographer.
A Ukrainian doctor was also killed in a separate artillery blast in Misrata on Wednesday. His wife was reportedly seriously injured in the incident.
Government spokesman Moussa Ibrahim has said that if foreign troops enter Misrata the government would “unleash hell”.
“We will be a ball of fire. We will make it 10 times as bad as Iraq,” he said, saying the government was arming people in preparation.
Hundreds of foreign workers, Libyans and injured people are being evacuated from Misrata by sea to the rebel-held city of Benghazi in the east.
UN Secretary General Ban Ki-moon has called on the Libyan authorities to “stop fighting and stop killing people”.
He said the UN’s priority was to bring about “a verifiable and effective ceasefire” to enable humanitarian work and political dialogue to take place.
US Secretary of State Hillary Clinton has condemned what she called the “vicious attacks” on Libyan civilians.
She also demanded that Libyan authorities immediately release US citizens they have “unjustly detained,” including at least two reporters.
The parents of Clare Gillis, one of the missing journalists, said she was able to contact them on Thursday for the first time since she was detained on 5 April.
They told the Atlantic, one of the papers Ms Gillis was working for, that she was in good health but had not been allowed a visit by humanitarian or diplomatic officials.