"Everything we do is defensive. We're protecting the allies. Russia is behaving aggressively. The annexation of Crimea is an aggressive behavior. Russia continues providing separatists in eastern Ukraine with modern weapons, anti-aircraft systems, sends troops there – this is an act of aggression. We protect our allies, Russia uses force to redraw the borders and destabilize Ukraine," Stoltenberg said, as reported by DW.

According to him, Russia is ready to use force against Ukraine, Georgia and Moldova.

"Russia is ready to use force against Ukraine, Georgia and Moldova. Russia more often mentions nuclear weapons as part of its defense strategy," NATO's chief said.

According to Stoltenberg, NATO is responding to new security challenges in a defensive way, which is proportionate and in line with the alliance's international commitments.

"We have increased the preparedness and readiness of our forces. We have strengthened our military presence on the eastern borders of the alliance. We have decided to set up a network of smaller headquarters in the Baltic States, Bulgaria, Poland and Romania. We have also stepped up our air patrols and presence in the eastern countries of the alliance in response to Russia's behavior," Stoltenberg said.

At this, Moscow now has a better-equipped and better-prepared army, he added.

"What is more, they conduct unexpected maneuvers to conceal aggressive actions against its neighbors. That is how Crimea was annexed," Stoltenberg said.

Therefore, by addressing these threats, NATO has been strengthening its military capabilities, being unprecedented since the end of the Cold War, according to Stoltenberg.