After one recent attack on a remote base in eastern Afghanistan, a check of the dead insurgents found evidence that the fighters were from three different factions, military officials said.

In the past, these insurgent groups have been seen as sharing ideology and inspiration, but less often plans for specific missions.

Now the intelligence assessments offer evidence of a worrisome new trend in which extremist commanders and their insurgent organizations are coordinating attacks and even combining their foot soldiers into patchwork patrols sent to carry out specific raids.

The change reveals the resilience and flexibility of the militant groups. But at the same time, officials say, the unusual and expanding alliances suggest that the factions are feeling new military pressure. American and NATO officials say these decisions by insurgent leaders are the result of operations by American, Afghan and allied forces on one side of the border, and by the Pakistani military — and American drone strikes — on the other.