(CNN)-- A band of Afghan insurgents breached a small U.S. outpost in southern Afghanistan early Tuesday, wounding nine coalition troops before all but one of the attackers were killed, two U.S. officials said.
The U.S. officials said at least eight insurgents somehow made their way into the security perimeter at Forward Operating Base Frontenac, in the Arghandab River valley. The area has been the scene of extensive insurgent activity in recent years.
Seven of the attackers were killed and the lone survivor was wounded, the officials said. Neither official could explain how the breach occurred, but initial reports indicate officials believe the insurgents might have had help from Afghan security personnel.
Tuesday's attack follows an incident Monday in which three gunmen in Afghan police uniforms fired on American troops in another location in southern Afghanistan, killing one and wounding several others.
Uniformed Afghans -- either insurgents in disguise or members of the country's police or military -- have been behind numerous killings of U.S. and NATO troops this year. The incidents have fueled mutual distrust between Afghan and allied forces in the now-decade-old conflict.