AJDABIYA, Libya – 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.

Tensions between the rebels and NATO were flaring even before the latest accident, with the fighters criticizing the alliance for doing too little to help them.

In a sign of the hair-trigger tensions along the front, thousands of civilians and fighters raced out of the rebel-held city of Ajdabiya in eastern Libya after reports that Moammar Gadhafi's forces gained ground in the chaos after the bombing. Some militiamen shouted insults against NATO as they retreated.

"We don't want NATO anymore!" cried fighter Basit bin Nasser. Another yelled: "Down, down with NATO."

In Brussels, NATO did not directly acknowledge responsibility for a blundered airstrike on the rebels, but noted that the area where the attack occurred was "unclear and fluid with mechanized weapons traveling in all directions."

"What remains clear is that NATO will continue to uphold the U.N. mandate and strike forces that can potentially cause harm to the civilian population of Libya," the alliance said in a statement.

But NATO faces the same challenges to avoid friendly fire deaths as commanders in other wide-ranging air missions such as Iraq and Afghanistan.