TRIPOLI - Libyan officials said on Sunday a NATO strike had hit a civilian house in the capital Tripoli, killing nine residents, and NATO said it was investigating the claim, which could sow new doubts about its mission.On another front in the four-month-old battle to force out Libyan leader Muammar Gaddafi, a doctor in the city of Misrata said eight rebel fighters had been killed and 36 wounded in fighting with government troops.