At least 13 rebels were killed when NATO- led aircraft bombed the area between Ajdabiya and Brega cities in the eastern part of Libya, rebel fighters said Saturday.
Another seven were injured. The circumstances of the attack were unclear, DPA reported.
NATO took over control from an impromptu coalition coordinated by the United States, with France and Britain playing leading roles.
Opposition forces on Friday moved to fortify the key north-eastern city of Ajdabiya, which is seen as an important stop between the capital Tripoli and the rebel stronghold Benghazi.
The city has been braced for a possible attack by leader Moamer Gaddafi's forces, as pro-democracy fighters struggled to recapture key eastern cities they lost earlier this week.
Al Jazeera reported Saturday that Gaddafi's forces have stopped using tanks because they can be easily spotted by NATO airplanes.
Instead they are using pick-up trucks with mortars on the back which look a lot more like the rebel forces.