Russia did supply separatists in eastern Ukraine with tanks and heavy weapons, U.S. officials confirmed on Friday, and has also filed a note of protest alleging Ukrainian forces crossed the border.
According to the State Department, Russia sent pro-Russian forces several military vehicles, including three T-64 tanks, as well as BM-21 "Grad" multiple rocket launchers, the New York Times reports.
"This is unacceptable," State Department spokeswoman Marie Harf said. "A failure by Russia to de-escalate this situation will lead to additional costs."
The tanks, no longer in use by the Russia military, were first discovered near Snizhne on Thursday.
The confirmation comes just as Russia sent Ukraine a note of protest for crossing the border in the Rostov region as forces took back the port city of Mariupol previously claimed by pro-Russian militants, according to NBC News. In a statement the Russian Foreign Ministry called crossing the border a "gross violation of fundamental rules of international law."
The Ukrainian forces were chased out of the Rostov region, but still kept control of the port city, which Ukraine has accused Russia of using to help funnel supplies to separatists.