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".
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Coal's share of the power mix had fallen to 7%, while wind climbed to new highs to provide 15% of the country's electricity.
State Department spokeswoman Heather Nauert said Pence "was ready to take this opportunity."
Opposition activists said government forces have brought in more reinforcements in recent days.