Russian President Vladimir Putin will suffer “massive consequences” for another invasion of Ukraine, NATO allies vowed in an unusual warning to Moscow.

“We are gravely concerned by the substantial, unprovoked, and unjustified Russian military build-up on the borders of Ukraine in recent months, and reject the false Russian claims of Ukrainian and NATO provocations,” the North Atlantic Council said in a Thursday rebuke. “Any further aggression against Ukraine would have massive consequences and would carry a high price.”