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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Sixty people died when a pick-up truck was blown up as they waited to collect permits and escorts a day after their home town was liberated.
Scotland Yard said the male youths will appear in custody at Westminster Magistrates' Court on Saturday.
Windy this weekend with gales across northwestern parts of the UK. Rain spreading southeast.