"In the last three days, a convoy of three T-64 tanks, several BM-21 or Grad multiple rocket launchers and other military vehicles crossed from Russia into Ukraine near the Ukrainian town of Snizhne. This is unacceptable," State Department spokesperson Marie Harf told reporters here, Xinhua reported.
She said Ukrainian President Petro Poroshenko lodged a protest with his Russian counterpart Vladimir Putin on Thursday, while US Secretary of State John Kerry voiced his concern with Russian Foreign Minister Sergei Lavrov a day earlier.
Russia, which denied sending troops or weapons to eastern Ukraine where armed militants are fighting the government forces, has made no comment on the movement of tanks.
US: We are concerned by new Russian efforts to support the separatists
"We are highly concerned by new Russian efforts to support the separatists," Harf said, urging Moscow to take advantage of a meeting between Ukrainian and Russian negotiators over the weekend to "demonstrate its commitments to peace" and help implement the peace plan mapped out by Poroshenko on June 7 in his inauguration speech.
The plan calls for a ceasefire, amnesty for militants to lay down their arms, safe conduct for Russian fighters back to Russia, and broad decentralisation of authority to Ukraine's regions.
"A failure by Russia to de-escalate the situation will lead to additional costs," Harf warned.