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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A man, reportedly armed with a knife and a baseball bat, was detained close to the event and police cleared the area as a precaution.
A remarkably mild night
Thousands of people have gathered in the centre of Manchester for a vigil in the wake of the terror attack.