The new Canadian commander of the NATO-led mission in Libya says that anyone there would be "ill advised" to continue attacking civilians, while pledging to investigate a report from the Vatican that dozens of civilians died in a coalition air strike.

Lt. Gen. Charles Bouchard said Thursday the alliance has taken over command of the international military force, and has 100 aircraft in operation to enforce the no-fly zone over much of the North African country.

There are also a dozen frigates patrolling off the Libyan coast in an effort to keep weapons from reaching either the rebels or Libyan leader Moammar Gadhafi's forces.

At a televised news conference at a NATO headquarters building in Naples, Italy, Bouchard said the handover of command from the U.S. was "seamless with no gaps in the effort." U.S. forces had been leading the UN-sponsored mission until the handover early Thursday morning.

Anyone attacking civilians "would be ill-advised to continue such activities," he said, adding that NATO has already flown 90 sorties over Libya.

Vatican's envoy in Libya claims that 40 civilians were killed by coalition airstrikes in Tripoli overnight, when a building collapsed.

Bouchard said he would probe the report, but noted that it would have taken place before NATO took command.

"I am aware of this news report," he told reporters. "I take every one of those issues seriously, but our mission began ... today."

NATO took command of the coalition force rebels seeking to oust Gadhafi's regime have suffered setbacks, losing ground they have gained since the foreign forces intervened in the fighting.

Libyan defectors

Hours before the handover, Gadhafi suffered a major diplomatic blow. British officials confirmed late Thursday that Moussa Koussa, the Libyan foreign minister, had resigned his position and defected.

Koussa, long considered one of Gadhafi's closest allies, has fled to Britain and has been debriefed by Western authorities.

British Foreign Secretary William Hague said the defection proves that the Gadhafi regime is "fragmented, under pressure and crumbling."

Guma El-Gamaty, a Libyan opposition organizer based in Britain, said that Koussa's resignation would be a "big hit" against the Gadhafi regime.

Koussa's departure follows the resignations of two other Libyan ministers who have joined the rebels fighting for Gadhafi's ouster.

Hague suggested that other defections could follow.

"Gadhafi must be thinking to himself, ‘Who will be the next to walk away?'" Hague said.

El-Gamaty said it is unlikely that Koussa would be welcomed into the opposition movement because of his history with the Gadhafi regime.

Hague said that Koussa was "not being offered any immunity from British or international justice," alluding to allegations that the former Gadhafi confidant played a key role in the 1988 bombing of Pan Am Flight 103 and other atrocities involving the Libyan regime.

El-Gamaty said Koussa "has been Gadhafi's right-hand man for years, running intelligence, running the Lockerbie bomber negotiations, running many things."

U.S., Britain mull arms for rebels

Inside Libya, Gadhafi's troops continued their grinding assault against rebel fighters who have been trying unsuccessfully to push towards the capital city of Tripoli for the past several weeks.

Rebel fighter Fathi Muktar said Gadhafi forces approached the gates of the strategic oil town of Brega on Thursday morning, after being pushed back the night before.

Muktar said "loads" of rebels were injured on the Brega front lines on Thursday, as they came under heavy shelling from Gadhafi's forces.

The rebels have been unable to build momentum as they have battled against Gadhafi's troops and hired mercenaries. While a no-fly zone has protected the rebels from regime air attacks, they are still being outgunned and overmatched on the ground.

The imbalance between the two sides has left some Western countries mulling the possibility of providing arms to the rebels, though NATO's chief said Thursday that he doesn't think the UN mandate for the ongoing operation would allow for that possibility.

NATO Secretary-General Anders Fogh Rasmussen told reporters in Stockholm that his organization was "there to protect the Libyan people, not to arm the people."

Britain and the United States have argued that existing UN Security Council resolutions on Libya could allow foreign governments to arm the rebels, despite the current arms embargo.

Fogh Rasmussen said he has "taken note of the ongoing discussions in a number of countries but as far as NATO is concerned…we will focus on the enforcement of the arms embargo."

Canadian role

Ottawa recently appointed Canadian Lieutenant-General Charles Bouchard as the NATO joint task force commander for the Libyan campaign and there are seven CF-18s deployed as part of the international air effort to enforce a no-fly zone over Libya.

Prime Minister Stephen Harper asserted on Thursday that Ottawa would not be putting Canadian soldiers on the ground.