French military jets are preventing forces loyal to Libyan leader Col Muammar Gaddafi from attacking the rebel-held city of Benghazi, French President Nicolas Sarkozy says.
It is believed to be the first act of intervention since the UN voted on Thursday for a no-fly zone over Libya.
Western and Arab leaders have been meeting in Paris to agree a course of action to confront Col Gaddafi.
“Our air force will oppose any aggression,” Mr Sarkozy said.
Hours earlier, pro-Gaddafi forces launched an assault on the Libyan rebel stronghold of Benghazi, a BBC journalist witnessed.
However, the Libyan government has denied it is attacking.
‘Stop the bombardment’
French aircraft have also flown over “all Libyan territory” on reconnaissance missions, French military sources said earlier.
The French Rafale jets took off from their base at Saint-Dizier in eastern France, a source told the Agence France-Presse news agency.
The planes encountered no problems during the first few hours of their mission, the source said, and the flights would continue for the next several hours.
Dutch Prime Minister Mark Rutte told journalists at the summit in Paris that he believed British, French and Canadian aircraft would launch the first airstrikes, the BBC’s Carole Walker in Paris reports.
Asked if those strikes would take place later on Saturday, Mr Rutte admitted that was a possibility, our correspondent says.
The new UN resolution authorises “all necessary measures” to protect Libyan civilians.
The international community was intervening to stop the “murderous madness” of Col Gaddafi, Mr Sarkozy said.
“In Libya, the civilian population, which is demanding nothing more than the right to choose their own destiny, is in mortal danger,” he warned. “It is our duty to respond to their anguished appeal.”
The rebels’ leader had earlier appealed to the international community to stop the bombardment by pro-Gaddafi forces.
A jet also appears to have been shot down over Benghazi. A rebel spokesman was quoted as saying the downed jet was a rebel plane which had been shot down by pro-government forces.
Reports from Benghazi suggest hundreds of cars packed with people were fleeing eastwards as fighting spread.
The United Nations refugee agency says it is preparing to receive 200,000 people fleeing the fighting, amid reports of hundreds of cars full of people heading for the Egyptian border, while others are attempting to flee on foot.
The first families had arrived at the Egyptian border, extremely frightened and traumatised, saying some of their homes have been completely flattened said UNHCR spokeswoman Elizabeth Tan.
However, the BBC’s Ben Brown, who is at the border, says so far there are a handful of families, in addition to the migrant workers who have been there since the crisis started.
UN Secretary General Ban Ki-moon has said the world must “speak with one voice” on Libya.