Ukrainian artillery destroyed a "significant" part of a Russian armored column that crossed into Ukraine during the night, President Petro Poroshenko told British Prime Minister David Cameron on Friday, according to the presidential website.
Ukrainian border guards and customs officials on Friday began inspecting a Russian humanitarian aid cargo for eastern Ukraine at a border point inside Russia, a Ukrainian military statement said.
A convoy of 280 Russian trucks brought the aid to the border in an operation which aroused Kyiv's suspicions that it might be used as a cover for a military operation in Ukraine. The aid is destined for eastern Ukraine, where a war between government forces and separatists has produced a humanitarian crisis.
Meanwhile, NATO accused the Kremlin on Friday of escalating the conflict in Ukraine.
The Russian government denied its troops had entered Ukraine, but the media reports risked further inflaming tensions between Moscow and the West, which have already imposed costly economic restrictions on each other.
"If confirmed, they are further evidence that Russia is doing the very opposite of what it's saying. Russia has been escalating the conflict, even as it calls for de-escalation," NATO spokeswoman Oana Lungescu said.
At a meeting of European Union foreign ministers in Brussels, British Foreign Secretary Philip Hammond said he was alarmed that Russian forces might have crossed the border.
"If there are any Russian military personnel or vehicles in eastern Ukraine, they need to be withdrawn immediately or the consequences could be very serious," he told reporters. Lithuania's foreign minister also voiced concern.