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.
More top news
Sadly, these high-school students have already received the most abrupt and cynical lesson of their lives.
A top temperature of 9 or 10 Celsius (48-50 F).
The poll comes ahead of a debate in the House of Commons on Friday on a new Bill which would introduce such a system in England.