The U.S. will not send ground forces to Libya, where armed rebels are fighting supporters of longtime leader Moammar Gadhafi, Defence Secretary Robert Gates says.

Gates's comments came just hours after NATO assumed full control of international air operations over Libya.

Gates told a congressional committee Thursday that the U.S. would limit its role, saying some other country could train the rebels trying to oust Gadhafi.

"My view would be, if there is going to be that kind of assistance to the opposition, there are plenty of sources for it other than the United States," said Gates. "Somebody else should do that."

Asked whether the U.S. involvement might inevitably mean "boots on the ground" in Libya, Gates replied, "Not as long as I am in this job."

Earlier Thursday, Stephen Harper reiterated that Canada was not "putting boots on the ground" in Libya.

CBC in Libya

Some of the rebel fighters injured in the clashes with pro-Gadhafi forces have been brought to a hospital in Ajdabiya, in eastern Libya.

(CBC)

Ibrahim Fahim was at the hospital, looking for comrades hit by a shell. He told CBC's Margaret Evans he is committed to going back out, despite Wednesday's dramatic rebel retreat in the face of Gadhafi's forces.

He told Evans the rebels have to continue their fight, saying Gadhafi "destroyed everything."

One man who was shot in the leg during clashes with Gadhafi loyalists thanked France, Italy and Canada for the airstrikes that helped knock out much of Gadhafi's air force, Evans said.

But Gadhafi's ground forces are still better armed than their opponents, and they made big gains in recent days, pushing the rebels out of the oil town of Ras Lanouf.

Evans said there are also reports that Gadhafi's forces are changing tactics and abandoning the slow-moving tanks that will draw the fire of allied planes, moving instead to pickup trucks and cars similar to those used by the rebels.

"But we are working very closely with our allies to enforce United Nations resolutions ... to try and see the departure of Mr. Gadhafi from power, because we've all been clear that he's lost legitimacy and he needs to go for the benefit and for the welfare of the people of Libya," said the Conservative leader said during an election campaign event in Halifax.

'Seamless' transition

Meanwhile, Lt.-Gen. Charles Bouchard, the Canadian commander of the NATO operation, said Thursday the transition of command from the U.S. had been "seamless with no gaps in the effort."

Bouchard warned forces attacking civilians in Libya that they would be "ill-advised" to continue such activities.

There were reports Thursday that pro-Gadhafi forces were shelling the city of Misrata, and the BBC said a group of retreating rebel fighters came under fire between Brega and Ajdabiya.

CBC's Nahlah Ayed said there has been a lot of back and forth between the two sides in recent days. On Wednesday, Gadhafi loyalists gained ground, pushing the rebels out of a key oil port.

The new commander also said NATO would investigate a claim by the Vatican's envoy in Libya that airstrikes in Tripoli during the night had killed 40 civilians, though he noted the alleged incident was said to have taken place before NATO took command.

"I take every one of those issues seriously," he said.

He said international forces are "very careful" when carrying out operations in Libya, and reiterated that NATO is operating within the legal mandate of UN Resolution 1973.

Gadhafi forces weakened

In Washington, Gates and the Joint Chiefs of Staff chairman, Admiral Mike Mullen, told the armed services committee that Gadhafi's military has been degraded by as much as 25 per cent, but Mullen noted that regime forces still outnumber the rebels by about 10-1.

"We have actually fairly seriously degraded his military capabilities, his air defence capabilities, his command and control capabilities," Mullen said.

"That doesn't mean he's about to break from a military standpoint, because that's just not the case" he said, noting that he has "great confidence" in NATO.

They also said that opposition groups are fractured and operating independently in different areas.

Gates said he believes political and economic pressures will eventually drive Gadhafi from power, but said the military operation will help force him to make those choices by degrading his defence capabilities.

Neither Gates nor Mullen provided any sort of timetable on how long the mission might take or how it would play out if Gadhafi doesn't step aside.

Gates declined to comment on the CIA activities in Libya. U.S. officials have acknowledged that the CIA has sent small teams of operatives into Libya and helped rescue a crew member of a U.S. fighter jet that crashed.

Earlier, NATO's secretary general said the alliance doesn't support U.S. and British suggestions that the UN mandate for the international military operation in Libya would allow arming rebels fighting Gadhafi's troops.

A Libyan rebel carries a machine-gun as fighters gather on the main road after retreating east, at the outskirts of the town of al-Agila, on Wednesday. A Libyan rebel carries a machine-gun as fighters gather on the main road after retreating east, at the outskirts of the town of al-Agila, on Wednesday. Nasser Nasser/Associated Press

Anders Fogh Rasmussen told reporters in Stockholm that NATO's position is "we are there to protect the Libyan people, not to arm the people."

The NATO operation — code named Unified Protector — includes enforcing the UN Security Council resolution mandating an arms embargo on Libya, enforcing a no-fly zone and protecting civilians from Gadhafi's troops.

NATO's governing body, the North Atlantic Council, has approved the alliance's operations for up to three months. That period could be extended if necessary, officials said.

Also Thursday, Britain refused to offer Libyan Foreign Minister Moussa Koussa immunity from prosecution after his apparent defection, but said his departure would hearten rebels fighting Gadhafi's regime..

With files from The Associated Press