The Russian strikes hit Syrian opposition fighters on Thursday at the al-Tanf crossing, which lies on Syria’s border with Iraq. The rebels there are battling the Islamic State, the Pentagon said, and are also supposed to be covered by a partial cease-fire that the United States and Russia brokered in February.

During the videoconference on Saturday, Pentagon officials “expressed strong concerns about the attack” on forces that are fighting the Islamic State, Peter Cook, the Defense Department spokesman, said in a statement.

American officials told the Russians that their strikes had continued even after they were officially informed of allied air support underway for the rebels and that this had “created safety concerns for U.S. and coalition forces,” the statement said.