The plan, codified in November in a 66-page document, stresses adaptability, increased readiness and investment to stay relevant.

At the meeting, NATO Military Committee chief Sir Stuart Peach cited Russia as the main threat in the Euro-Atlantic region, but noted that China is also a potential danger.

"Since 2014 [the year of Russia's annexation of Crimea], we have implemented the biggest reinforcement of collective defense in a generation. We have strengthened our military posture from the Baltic to the Black Sea," Peach said at the beginning of Tuesday's meeting.

"Although we have seen some reduction in the number of Russian troops near the border [with Ukraine], tens of thousands remain, including their weapons and equipment," he said.

Peach also noted that NATO faces different future challenges and recommended an analysis of the potential consequences of climate change.

"Climate change will impact our lives in many ways, but crucially for the Military Committee, we are focused on how it affects our common security," Peach added. "From there, the military authorities can further integrate climate change risks and considerations into NATO's military planning and exercises."

The United States was represented at the meeting by Army Gen. Mark Milley, chairman of the Joint Chiefs of Staff.

The conference of defense chiefs was a prelude to a planned summit of NATO heads of state in June.

NATO Secretary General Jens Stoltenberg said Tuesday that the summit ''will be a unique opportunity to future-proof our Alliance, reinvigorate the transatlantic bond and demonstrate our commitment to transatlantic solidarity. Not just in words, but in deeds."

He offered eight core proposals for discussion in June, aimed at "reinforcing the unity between Europe and North America, broadening NATO's approach to security and safeguarding the rules based international order."

They include closer political consultations and a renewed commitment to collective defense; measures to step up work on resilience, climate change and new technologies; and better coordination with democracies around the world.