An apparent NATO airstrike slammed into a rebel combat convoy Thursday, killing at least five fighters and sharply boosting anger among anti-government forces after the second bungled mission in a week blamed on the military alliance.

The attack — outside the strategic oil port of Brega — brought fresh questions about coordination between NATO and the patchwork of rebel militias in a conflict described by a senior U.S. commander as a stalemate that could eventually require the Pentagon to reassert more power, and possibly even send in ground forces.