NATO forces last year intervened in Libya following a U.N. Security Council resolution authorizing military action to protect civilians from attacks by forces loyal to Moammar Gadhafi.
Human Rights Watch, in a 76-page report, said two dozen children and nearly as many women were among 76 civilian deaths in Libya attributed to NATO airstrikes.
Fred Abrahams, a special adviser at Human Rights Watch, said that while NATO took steps to minimize civilian casualties during last year's campaign, outstanding issues remain.
"Attacks are allowed only on military targets and serious questions remain in some incidents about what exactly NATO forces were striking," he said in a statement from Brussels.
The rights organization said it visited eight questionable strike sites in Libya and determined seven of them lacked a clear military target.
A U.N. commission of inquiry in March found that pro- and anti-government forces likely committed war crimes and crimes against humanity during last year's war.
The commission said there were widespread acts of torture, unlawful killing and rape during the Libyan conflict. The report added that while NATO forces conducted a "highly precise campaign," there were confirmed civilian casualties inside Libyan targets that had no evidence of military utility.