NATO Secretary General Jens Stoltenberg says Russia continues to maintain massive invasion force on the border with Ukraine, ready to attack.

"But so far, we do not see any sign of de-escalation on the ground. No withdrawals of troops or equipment. This may of course change. However, what we see today is that Russia maintains a massive invasion force ready to attack. With high-end capabilities. From Crimea to Belarus. This is the biggest concentration of forces in Europe since the Cold War," he said at a press conference in Brussels on Wednesday after the first day of the North Atlantic Council meeting at the level of defense ministers.

Stoltenberg said NATO allies made it clear that further Russian aggression against Ukraine would have a high cost.

"And we have called out Russia's actions, plans and disinformation. At the same time, NATO remains prepared for dialogue. It is not too late for Russia to step back from the brink of conflict. And choose the path of peace," the Alliance Secretary General said.

However, he said NATO will not compromise on core principles, in particular the right of each nation to choose its own path, as well as the ability to protect and defend all allies.

"We have already enhanced our deterrence and defence. With more troops, planes and ships. And higher readiness of the NATO Response Force. These steps are defensive. NATO is not a threat to Russia," Stoltenberg said.

At the same time, the Secretary General said the Alliance "does not know what will happen in Ukraine."

"But the situation has already demonstrated we face a crisis in European security. Moscow has made it clear that it is prepared to contest the fundamental principles that have underpinned our security for decades. And to do so by using force. I regret to say that this is the new normal in Europe," Stoltenberg said.