Nato said its new satellite images raise suspicions about Russia's role in moving military equipment into eastern Ukraine.
The images showed Russian tanks arrived at a staging area close to the Ukraine border days before similar tanks appeared in areas of eastern Ukraine under separatist control, Nato said. Where the tanks came from is in dispute.
Ukrainian Interior Minister Arseny Avakov said the tanks had rolled across the border from Russia, but rebels said they were from a Ukrainian military stockpile.
Scores of people in the region have been killed in fighting between government forces and pro-Moscow separatists.
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Western areas will see the rain first but by the end of the day it will also have spread eastwards.