US Secretary of State John Kerry warned Russia that it must not intervene in Ukraine under the "pretext of peacekeeping", his department said.
The statement follows Russia's version of the call, in which its Foreign Minister Sergei Lavrov was said to have called for urgent measures to avert a "humanitarian catastrophe" in the east of the country.
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The success of the No camp is mainly due to Tsipras convincing people a No vote will strengthen their hands in negotiations.
As we move into the weekend, expect things to get rather more unsettled weather-wise - thought it will stay hot and humid for some.
More than 80 prominent Britons, including Boris Johnson, Russell Brand and former Government ministers, urge Obama to release Aamer.