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 lucky football fan is heading to this weekend's FA Cup Final at Wembley after picking which had been fired into space.
Ciaran Williamson and two friends were hit by the gravestone while playing in Craigton cemetery in the Cardonald area of Glasgow.
Attorney General Loretta E. Lynch spoke at a press conference earlier following arrests made in Zurich.