NATO as accused Russia of breaching Ukraine’s sovereignty by directly supporting rebels in the east of the country.
Secretary-General Anders Fogh Rasmussen, speaking after an emergency meeting with NATO ambassadors, said that "despite hollow denials", it was clear that Russian forces illegally crossed the border into Ukrainian territory.
"This is not an isolated action, but part of a dangerous pattern over many months to destabilise Ukraine as a sovereign nation," he said.
"This is a blatant violation of Ukraine's sovereignty and territorial integrity. It defies all diplomatic efforts for a peaceful solution."
Rasmussen called on Russia to halt all military action and supporting the rebels, and to take “immediate and verifiable steps towards de-escalation of this grave crisis".