The United States says it can't confirm reports of a new Russian military incursion in eastern Ukraine despite claims by the Kiev government.
A Ukrainian military spokesman said on Friday that a column of 32 tanks, 16 howitzer artillery and 30 truckloads of troops crossed into the separatist-controlled Lugansk region.
US State Department spokeswoman Jen Psaki admits Russian battle tanks, armoured vehicles and cargo trucks had been seen on Thursday at a rail yard about 25 kilometres from the border.
She said the United States did not have "independent confirmation" of the latest reports but voiced Washington's dismay.
"If confirmed, the United States condemns this most recent incursion into Ukrainian territory," she said.
"It would be another blatant violation of the Minsk agreement signed by Russia and the separatists."
Kiev said five soldiers were killed in the latest bout of fighting despite a ceasefire with the rebels, and fears grew of a return to all-out fighting.
Washington for months has denounced the presence of Russian soldiers and weapons in eastern Ukraine in support of separatist forces.
"Russian forces and weapons must be withdrawn from Ukraine, Ukrainian sovereignty must be restored along the Ukrainian side of the international border, and that border needs to be monitored by the OSCE, and all hostages must be released immediately," Psaki said, referring to the Organisation for Security and Co-operation in Europe.
