DONETSK, Ukraine — The State Department said Friday that Russia had sent tanks and other heavy weapons to separatists in Ukraine, supporting accusations Thursday by the Ukrainian government.

A convoy of three T-64 tanks, several BM-21 multiple rocket launchers and other military vehicles crossed the border near the Ukrainian town of Snizhne, State Department officials said. The Ukrainian Army reported Friday that it had destroyed two of the tanks and several other vehicles in the convoy.

“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.”

Overnight Friday, separatists using antiaircraft and heavy machine guns fired on a military transport plane as it was landing in Luhansk, the Ukrainian Defense Ministry said. It did not give details of casualties, but offered condolences to family members of the victims. News reports said the plane was carrying as many as 49 military personnel.