A senior Pakistani defense official acknowledged that Pakistani troops fired first, sending a flare, followed by mortar and machine-gun fire, toward what he said was “suspicious activity” in the brush-covered area below their high-altitude outpost barely 500 yards from the border.

According to Afghan security officials, their commandos were engaged with U.S. Special Operations troops in a nighttime raid against suspected Taliban insurgents when they came under cross-border fire and called in an airstrike.