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
Odense zoo plans to publicly dissect a year-old lion - undeterred by outrage at giraffe who was killed in Copenhagen last year.
The Duke and Duchess of Cambridge met young people who have battled mental health issues on Saturday.
Former Chancellor Lord Geoffrey Howe was Margaret Thatcher's longest-serving Cabinet minister, also taking on the role of Foreign Secretary.