Washington has urged Moscow to stop its "extremely dangerous and provocative" activities after Ukraine said it had destroyed part of a Russian military convoy that entered its territory.
US National Security Council spokeswoman Caitlin Hayden warned yesterday that "the escalation in Russian activity designed to destabilise Ukraine in recent weeks is extremely dangerous and provocative."
NATO accused Russia of active involvement in the "destabilisation" of eastern Ukraine, where pro-Kremlin separatists have been fighting against Kiev for four months.
Ukrainian President Petro Poroshenko told British Prime Minister David Cameron that government artillery had destroyed a "considerable part" of a small military convoy that entered the country, the presidency said in a statement.
Moscow's defence ministry dismissed the alleged military column as a "phantom," its latest denial of Western accusations that it is funnelling weapons to the pro-Russia separatists who launched an insurgency against Kiev in April.
Adding to the diplomatic pressure on Moscow, the European Union demanded that Russia "put an immediate stop to any form of border hostilities, in particular to the flow of arms, military advisers and armed personnel into the conflict region, and to withdraw its forces from the border."
German Chancellor Angela Merkel voiced that view in a phone call Friday with Russian President Vladimir Putin, while French President Francois Hollande called on Moscow to respect Ukraine's territorial integrity and for both sides to try to de-escalate the "very high tensions."
Kiev and Moscow have been wrangling for days over a larger Russian convoy that Moscow says is carrying humanitarian aid for besieged rebel-held cities but which Kiev suspects could be a "Trojan horse" to provide military help to the insurgents.
But Russian Defence Minister Sergei Shoigu "guaranteed" his US counterpart Chuck Hagel that there were "no Russian military personnel involved in the humanitarian convoy, nor was the convoy to be used as a pretext to further intervene in Ukraine," the Pentagon said.
Russian officials tried to prove that by showing off the contents of 10 lorries with baby formula, rations and bottled water to journalists.
In Washington, Hayden insisted that "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."
NATO chief Anders Fogh Rasmussen backed reports of the "Russian incursion" after British media said it had seen the column of some 20 military vehicles cross the border on Thursday.
"It just confirms the fact that we see a continued flow of weapons and fighters from Russia into the eastern Ukraine," he said.