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.
More top news
A&E services at County Hospital in Stafford have been suspended for under 18s because the service is 'not clinically safe'.
GCSE grades have fallen across the board but girls continue to outperform boys, according to this year's results.
The serial killer Peter Sutcliffe has been moved from Broadmoor psychiatric hospital to prison.