MEXICO CITY — The Biden administration said Monday that Iran’s sale of lethal drones to Russia for use in its ongoing invasion of Ukraine means the country may be “contributing to widespread war crimes.”

National Security Adviser Jake Sullivan leveled the charge against Iran on Monday as he spoke to reporters accompanying President Joe Biden on a trip to Mexico. While it did not signal a policy shift, the charge marked some of the sharpest U.S. rhetoric against Iran since it began providing weapons to Russia to support its nearly year-long war in Ukraine. It comes as the U.S. and European partners are looking to further ostracize both nations in the court of public opinion, as they face challenges with physically stopping the transfers of weapons on which Russia is increasingly reliant.

Sullivan said Iran had chosen “to go down a road where their weapons are being used to kill civilians in Ukraine and to try to plunge cities into cold and darkness, which from our point of view, puts Iran in a place where it could potentially be contributing to widespread war crimes.”

Sullivan pointed to European and U.S. sanctions on Iran put in place after the U.S. exposed Iran’s weapons sales to Russia last year as examples of how they are trying to “make these transactions more difficult.” But he acknowledged that “the way that they are actually carrying them out physically makes physical interdiction a challenge.”