TRIPOLI — NATO officials said it was possible an air strike carried out in Tripoli early Sunday killed noncombatants, an acknowledgment that is likely to fuel the war of words the alliance has been waging with the Libyan government over civilian casualties. NATO officials said they regretted “any loss of civilian lives” in a strike in the northern district of Souk Jouma following an operation targeting what the alliance descried as a “missile site” used by Gaddafi’s forces. As NATO’s campaign in Libya enters its fourth month, the alliance has faced mounting questions about the pace and purpose of its mission in Libya.