* Civilian casualties a growing concern for Western forces
* NATO makes 180 sorties over Libya on first day
By Justyna Pawlak
BRUSSELS, April 1 (Reuters) - Bad weather and mounting concern over civilian casualties curbed NATO operations in Libya in the first day since it assumed command of the Western campaign against Muammar Gaddafi, NATO officials said on Friday.
NATO took over air strikes targeting Gaddafi's military infrastructure as well as enforcement of a U.N.-mandated no-fly zone and arms embargo on Thursday, replacing a coalition led by the United States, France and Britain.
Since then, military analysts say there have been few signs of the intense air strikes that dominated the beginning of the nearly two-week, Western-led campaign.
NATO officials said sandstorms had an impact on initial operations, limiting the alliance's ability to identify air strike targets, although the weather was improving on Friday.
"Yesterday, we were somewhat restricted by bad weather," one NATO official said.
However, the campaign had also entered a more "challenging" phase, he said, with forces loyal to Gaddafi operating increasingly in populated districts as they mounted an intense attack on rebel-held territory in Libya.
"There is no doubt it will be more challenging for us to identify targets of military forces that are attacking civilians."
More reports of civilian casualties of air strikes emerged on Friday, although there was no official confirmation.
A senior Vatican official in the Libyan capital, quoting what he called reliable sources in close contact with residents, said on Thursday 40 civilians had been killed in Tripoli.
And on Friday BBC television quoted a Libyan doctor saying a coalition strike had killed seven civilians near the oil town of Brega. NATO was making inquiries but had no confirmation.
Civilian deaths haunt the calculations of coalition governments. Any sign of mounting casualties could shatter a fragile consensus between Western and Arab capitals who first called for creation of a militarily enforced no-fly zone.
Military experts said NATO would continue to face constraints in the coming days in identifying targets that would steer clear of civilian areas, after much of Gaddafi's heavy infrastructure was damaged in the early days of the campaign.
"They (Western-led forces) captured all of what we may call the low-hanging fruit, the armoured columns, those (targets) in obvious positions on open roads, sitting on open terrain," said Shashank Joshi of the Royal United Services Institute.
"What we may now we left with is ... heavy weaponry on the ground that's more difficult to find and isolate because it is next to urban targets," he said.
In a sign of continued strength, Gaddafi's troops have succeeded in the past few days in pushing back rebels who were trying to press westward along the Mediterranean coast from their stronghold of Benghazi towards Tripoli.
Admiral Mike Mullen, the top U.S. military officer, said on Thursday Gaddafi was far from beaten. "We have actually fairly seriously degraded his military capabilities ... That does not mean he is about to break, from a military standpoint."
NATO said at least 14 of its 28 member states were taking part at the start of the military campaign, with their forces making about 180 sorties that included fighter jets, AWACS surveillance planes and support aircraft to enforce the no-fly zone.
NATO officials would not specify what targets, if any, were hit on Thursday and Friday. U.S. and British ships and submarines fired more than 100 Tomahawk cruise missiles in one night at the start of the operations in March..
Initial NATO planning envisages a 90-day operation, but the alliance could extend or shorten it if necessary.
For full Reuters coverage of Libya, please see.
Additional reporting by William Maclean in London; editing by Mark Heinrich