BRUSSELS (AFP) - NATO defended the credibility of its air war in Libya on Tuesday after Italy warned that civilian deaths could undermine the campaign amid growing signs of strains within the alliance. Italian Foreign Minister Franco Frattini warned that NATO's credibility was "at risk" following the alliance's first civilian casualties, and urged it to ensure it was not providing ammunition to Kadhafi's propaganda war."We cannot run the risk of killing civilians. This is not good at all," Frattini said Monday at a meeting of European foreign ministers. The blunder -- an embarrassment for a mission that prides itself on protecting Libya's people from the regime -- came on the heels of a friendly fire incident last week in which a column of rebel vehicles were hit by NATO warplanes. Bracken rejected a regime claim of further civilian casualties, however, reiterating that an air strike in the western Tripoli suburb of Sorman on Monday hit a legitimate military target. "If you look at our track record, we have taken utmost care to avoid civilian casualties and we will continue to do so," said NATO spokeswoman Oana Lungescu. But Frattini's warning added to strains emerging within the alliance as the war drags on, with resilient Kadhafi refusing to step down while rebels struggle to defeat a better-trained army. British Prime Minister David Cameron butted heads with his own brass over the campaign Tuesday after top officers warned that the military is overstretched. After speaking with the heads of the army and navy, Cameron said: "They are absolutely clear that we are able to keep up this mission for as long as is necessary, and that time is on our side, not on Kadhafi's side." NATO Secretary General Anders Fogh Rasmussen and outgoing US Defence Secretary Robert Gates have pressed allies to step up their roles in the mission to relieve nations like Britain that have borne the brunt of the work.