Britain summoned Russia's ambassador to clarify reports of a Russian military incursion into Ukraine and a build-up of equipment near the border, the Foreign Office said.
Russian ambassador Alexander Yakovenko was summoned following reports that a small column of Russian armoured vehicles had crossed overnight into an area of Ukraine where pro-Moscow rebels are battling government forces.
Nato has accused the Kremlin of escalating the conflict in Ukraine.
More top news
A number of Britons are feared stranded on Mount Everest after the Nepal earthquake triggered a deadly avalanche on Saturday.
Prime Minister defends royals' decision to give birth in a private hospital - instead of going with the NHS
Tech giants Facebook and Google both have simple, powerful tools to help friends and family keep track of loved ones