In his opening remarks at a meeting of Nato defence ministers, Secretary General Jens Stoltenberg describes the current situation as "a dangerous moment for European security", APA reports citing BBC.
"Russia has amassed an invasion force on the borders of Ukraine; at the same time there are signs from Moscow that diplomacy should continue - this gives grounds for cautious optimism," he says.
The secretary general cautioned there had been no sign of de-escalation on the ground.
He reiterates that Nato "is not a threat to Russia" and that the military alliance remains ready to talk "and find a diplomatic way forward".
But he adds: "While we continue to work for the best, we must also be prepared for the worst.
"We will do what is necessary to protect and defend all allies."