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
An astronaut on the International Space Station has shared his experiences from outer space in the first Twitter chat from the craft.
Kate is now an advanced diver after passing a Professional Association of Diving Instructors (PADI) advanced open water diver qualification.
The Queen and other members of the Royal family have been photographed in striking "quadruple" portraits.