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.”

Ukrainian anxiety about a new Russian assault has fluctuated with waves of Russian military exercises this year, only to crescendo in recent weeks as Russian military movements intersected with a manufactured migrant crisis in Belarus. Russian officials, who deny invading Ukraine and pretended not to have deployed Russian special forces to annex Crimea, maintain that the security impasse can only be broken by a Western pledge not to expand NATO and other security guarantees for Moscow.

“Nothing has changed," Ukrainian Security and Defense Council Secretary Oleksiy Danilov told Reuters. "There has been some [increase in troop numbers] but not critical enough for us to say, 'This is it, [an invasion] is going to happen now.'”

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Putin has sent more than 90,000 troops to the vicinity of Ukraine, according to Danilov, but that figure is well below the half-million that Ukrainian officials estimate would be necessary for a proper invasion. Against that backdrop, Secretary of State Antony Blinken’s top lieutenant for European and Eurasian Affairs traveled to Moscow for meetings with Russian officials.

“They had a detailed discussion of security guarantees in the context of the persistent attempts by the US and NATO to change the European military and political situation in their favor,” the Russian Foreign Ministry said in a summary of Russian deputy foreign minister Sergey Ryabkov’s Wednesday conversation with State Department assistant secretary Karen Donfried.

Ukrainian officials maintain that they will not allow Russia to dictate their choice of allies. "We will not take orders from anyone, we will not tolerate any tsar, we are a different kind of people,” Danilov said. “Is compromise possible? Yes, it is possible, but not at the expense of our independence and our country.”

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NATO echoed that point Thursday while warning Putin not to endanger the trans-Atlantic allies.

“We call on Russia to immediately de-escalate, pursue diplomatic channels, and abide by its international commitments on transparency of military activities,” the North Atlantic Council said. “We are seriously assessing the implications for Alliance security of the current situation. We will always respond in a determined way to any deterioration of our security environment, including through strengthening our collective defense posture as necessary."