A convoy of three T-64 tanks, several BM-21 “Grad” multiple rocket launchers and other military vehicles crossed the border near the Ukrainian town of Snizhne, State Department officials said. Reports and images of the weapons’ presence circulated on Thursday, but there were conflicting claims about where they had come from.

“This is unacceptable,” said Marie Harf, the deputy State Department spokeswoman. “A failure by Russia to de-escalate this situation will lead to additional costs.”

A Western official said that intelligence about the movement of the tanks and other weapons into Ukraine was shared on Friday with NATO allies. Secretary of State John Kerry complained earlier this week about the flow of Russian arms to separatists in Ukraine in a phone call to Sergey V. Lavrov, the Russian foreign minister.