Nato said it "seamlessly" assumed full command of military operations over Libya today, and warned combatants on the ground against attacking civilians.
Nato had agreed on Sunday to take over all operations from a coalition led by the United States, France and Britain. The move puts the 28-nation alliance in charge of air strikes that have targeted Libyan leader Muammar Gadafy's military infrastructure, and of policing a no-fly zone and an arms embargo.
"The transition has been seamless, with no gaps," Canadian Lieutenant-General Charles Bouchard, commander of Nato's Libya operations, told reporters at the military alliance's Southern European headquarters in Naples, Italy.
"Nato is fully responsible," he said.
Since Nato assumed control this morning, its aircraft had conducted more than 90 flights, and the alliance had more than 100 fighter jets and support aircraft at its disposal as well as a dozen frigates to control the Mediterranean, he said.
However, despite nearly two weeks of Western air strikes, Col Gadafy's troops have succeeded in the past few days in pushing back rebels who were trying to press westward along the coast from their stronghold of Benghazi towards the capital Tripoli.
Lieut-Gen Bouchard said: "I would like to offer closing thoughts for those who are acting against civilian populations in civilian centres: you would be ill-advised to continue such activities.
"I recommend that you cease these activities."
The no-fly zone was imposed under a UN mandate authorising "all necessary measures" to protect civilians from attack by Col Gadafy's forces.
Nato officials say alliance planning foresees a 90-day operation, but the timetable will depend on the United Nations.
Nato also said it was taking seriously, and investigating, a Vatican complaint about civilian casualties in Western air strikes in Libya.
A senior Vatican official in the Libyan capital, quoting what he called reliable sources in close contact with residents, said at least 40 civilians had been killed in Tripoli.
"It is a news report and I appreciate the source of this report but it is worth noting that I take every one of those issues seriously," Lieut-Gen Bouchard told reporters.
"We are very careful in the prosecution of any of the possible targets that we have. We have very strict rules of engagement provided to us and we are operating within the legal mandate of our United Nations mandate," he added.
Western powers say they have no confirmed evidence of civilian casualties.
Reuters