The U.S. State Department accused Russia on Friday of sending tanks and other heavy military equipment to separatists in Ukraine, marking a significant escalation in a conflict that had seemed to be simmering.

"Russia will claim these tanks were taken from Ukrainian forces, but no Ukrainian tank units have been operating in that area," deputy spokesperson Marie Harf said in a statement Friday. "We are confident that these tanks came from Russia."

The escalation was "unacceptable," Harf said, warning of further potential "costs" on Russia.

She said Russia had sent convoy of three T-64 tanks, several BM-21 or Grad multiple rocket launchers, and other military vehicles into Ukraine over the past three days. The T-64 tank is no longer used by Russian forces, but they have been kept at a deployment site in southwest Russia and were recently deployed.

"We assess that separatists in eastern Ukraine have acquired heavy weapons and military equipment from Russia, including Russian tanks and multiple rocket launchers," Harf said.

Until the past few days, the crisis in Ukraine had seemed to be calming down. One week ago, U.S. President Barack Obama had a conversation with Russian President Vladimir Putin that left U.S. officials "optimistic" about de-escalation.

A video that made its way online Thursday showed a tank advancing toward Donetsk, the city in Ukraine where pro-Russian separatists have stirred angst and taken over government buildings.



Putin spoke Thursday with newly inaugurated Ukrainian President Petro Poroshenko, who called to object to the reported advancement of tanks toward the hands of pro-Russian separatists. A spokesman for Poroshenko wrote on his Facebook page that Poroshenko told Putin the escalation was "unacceptable."

The new developments led U.S. Sen. Bob Menendez (D-N.J.), the chair of the Senate Foreign Relations Committee, to call on Obama to impose new sanctions on Russia.

"In my view, unless Putin is confronted by stronger disincentives, he will not cease support for the insurgents who are occupying more and more Ukrainian territory along the Russian border," Menendez wrote in a letter to Obama. "He will continue to seek to ensure that the Ukrainian government can’t stabilize the situation or address the pressing needs of the Ukrainian public."