NATO's operational commander is warning forces attacking civilians in Libya that they would be "ill-advised" to continue such activities.
Lt. Gen Charles Bouchard, a Canadian now heading the international operation, says the transition of command from the U.S. had been "seamless with no gaps."
Gen. Bouchard briefed reporters from his command centre near Naples on Thursday, his first day as commander of the international operation.
Gen. Bouchard said he was "aware" of news reports citing a Tripoli-based Italian Catholic bishop who said that 40 civilians died when a building collapsed during a bombing in the Buslim district of Tripoli.
"We are investigating and we will report the details once the investigation is completed," Bouchard told reporters via videolink from his headquarters in Naples, Italy.
"My investigation is to ascertain whether or not NATO forces were involved in this incident," he said.
Forces loyal to Libyan leader Moammar Gadhafi shelled the rebel-held city of Misrata on Thursday and dozens of civilians have been killed in the past few days when their homes were hit, a rebel spokesman said.
Misrata, the last big rebel strong-hold in western Libya, has been encircled by pro-Gadhafi forces for weeks and repeated Western air strikes aimed at protecting civilians there have not so far succeeded in halting the attacks.
"Massacres are taking place in Misrata," the rebel spokesman, called Sami, told Reuters by telephone.
"Artillery bombardment resumed this morning and is still going on. The (pro-Gadhafi) brigades could not enter the town but they are surrounding it," he said.
"Twenty civilians were killed yesterday after their houses were hit by bombardments. Many people were wounded." That death toll added to the dozens who, residents say, have been killed in fighting over the past 10 days.
Accounts from Misrata could not be independently confirmed because Libyan officials have prevented foreign journalists from reporting freely in the city.
Libyan officials deny attacking the civilians, but say they are trying to root out terrorists who have been holding the local population hostage.
Mr. Gadhafi will stay in the country "until the end" to lead it to victory against its enemies, a government spokesman said on Thursday.
Speaking after former Foreign Secretary Moussa Koussa defected and flew to Britain on Wednesday, the spokesman said Western air strikes against Libya had only united its top leadership against "a clear enemy."
"If this aggression did anything, it only rallied people around the leader and the unity of the nation," Mussa Ibrahim said in Tripoli. "Especially now. They see a clear enemy."
Asked if Gadhafi and his sons were still in the country, he said: "Rest assured, we are all here. We will remain here until the end. This is our country. We are strong on every front."
U.S. Defense Secretary Robert Gates said on Thursday Mr. Gadhafi will likely be removed from power over time by his own people, as a result of political and economic measures,
But Mr. Gates, in prepared remarks to Congress, stressed that removing Gadhafi was "not part of the military mission" by coalition forces acting under a UN Security Council mandate.
The comments came a day after revelations that President Barack Obama signed a secret order authorizing covert U.S. government support for rebel forces seeking to oust Gadhafi.
With files from The Associated Press and Agence-France Presse