US secretary of state John Kerry has urged Russia to support efforts to free military observers held by pro-Moscow forces in Ukraine.
Mr Kerry also told Russian foreign minister Sergei Lavrov in a phone call that the United States was concerned that Moscow's actions were "undermining stability, security and unity" in Ukraine.
The US State Department said Mr Kerry cited Russia's "provocative" troop movements along the border, Moscow's support for separatists and its "inflammatory rhetoric".
More top news
The PM promises to double to 200,000 the number of homes built under an existing scheme aimed at easing Britain's housing shortage.
The Met Office has weather warnings in place for snow and ice until 10am in the north and west.
The plan, which would effectively strip banks of the benefit of recent corporation tax cuts, forms part of the party's election manifesto.