French warplanes have shot down a Libyan plane in the first incident of its kind since enforcement of the UN no-fly zone began, a US official said.
The incident happened near the besieged western city of Misrata, reports said.
Dozens of coalition missiles have already hit military bases, with the aim of ending Col Muammar Gaddafi's ability to launch air attacks.
UK officials said on Wednesday that Libya's air force no longer existed as a fighting force.
Coalition forces have pounded Libyan targets for a fifth consecutive night.
The French military said their planes had hit an air base about 250km (155 miles) south of the Libyan coastline, in an incident apparently unrelated to the shooting down of the Libyan plane.
French officials did not give any further information on the location of the target or the damage.
One US official quoted by Associated Press news agency said the Libyan plane shot down by France was a G-2/Galeb, a training aircraft with a single engine. The French plane involved was a Rafale fighter, the same US official said.
Fresh fighting has meanwhile been reported in Misrata, scene of a bitter battle for control which has lasted for many days.
Misrata resident Muhammad told the BBC many large explosions were heard overnight in the city.
"Even now we continue to hear the aeroplanes circling the air above Misrata," he said.
"Gaddafi's forces have occupied the main street - there are snipers all along the rooftops of that street. They are firing indiscriminately into the main street and the back streets.
"But the heavy artillery and shelling has stopped since yesterday [Wednesday]. In that sense, we are in a much better position."
Further east in the strategically important city of Ajdabiya, residents described shelling, gunfire and houses on fire.
Nato members have been holding talks about assuming responsibility for the no-fly zone over Libya, so far without agreement.
Turkey is an integral part of the naval blockade, but has expressed concern about the alliance taking over command of the no-fly zone from the US.
UN Secretary General Ban Ki-moon has urged all sides in Libya to cease hostilities. "All those who violate international humanitarian and human rights law will be held fully accountable," his spokesman Martin Nesirky said.