The White House said on Friday it could not confirm reports that Ukraine had disabled vehicles in a Russian convoy inside Ukraine and warned Moscow that any intervention into Ukraine without Kiev's permission was unacceptable.
"Even as we work to gather information, we reiterate our concern about repeated Russian and Russian-supported incursions into Ukraine, "White House spokeswoman Caitlin Hayden said in a statement.
"Russia has no right to send vehicles, persons, or cargo of any kind into Ukraine, under any pretext, without the government of Ukraine's permission," she said.
Ukraine said its artillery destroyed part of a Russian armoured column that entered its territory overnight and said its forces came under shellfire from Russia on Friday in what
appeared to be a major military escalation between the ex-Soviet states.
Russia's government denied its forces had crossed into Ukraine.
The White House said Russia had engaged in provocative behavior to hurt Ukraine and urged President Vladimir Putin to stop supporting separatists.
"The escalation in Russian activity designed to destabilize Ukraine in recent weeks is extremely dangerous and provocative. It includes supplying separatist fighters with tanks, armored vehicles, artillery, and multiple rocket launchers (MRLs)," Hayden said in the statement.
"Yesterday, President Putin pledged to 'do everything in (Russia's) power' to end the conflict in Ukraine. He can start by ending Russian firing into Ukraine; stopping its supply of
weapons, support and cash to separatists; and closing the border to militants," she added.