The U.S. military used helicopter gunships, warplanes and artillery to kill what it said were members of an Iran-backed militia that had fired rockets at a base used by American forces in Syria, in the second retaliatory strike in two days.

Clashes with Iranian-backed paramilitaries in Syria are likely to heighten tensions with Tehran at a crucial point for negotiations between the U.S. and Iran over reviving the international agreement that puts limits on Tehran’s nuclear ambitions in exchange for sanctions relief.