"All together we expect this force to be 40,000 strong," Stoltenberg said during a briefing.
Stoltenberg announced the alliance’s intentions to increase the number of servicemen in its rapid response forces on Monday. The additional troops will include Special Forces and rapid response teams, as well as marine and air units.
The bloc agreed to establish a rotational NATO presence in Eastern Europe and the Baltic states at its summit last September in Wales. Some NATO allies, including the Baltic States and Poland, have requested a permanent military presence by the bloc.
NATO members will create a Very High Readiness Joint Task Force (VJTF), dubbed "Spearhead," to have a deployable force available to respond to threats on short notice. According to Stoltenberg, the new force will become operational next year.
Russia has repeatedly expressed its concerns over the buildup of NATO forces in Eastern Europe, which started amid the crisis in Ukraine. The bloc has accused Moscow of meddling in the country’s internal affairs, a claim Moscow denied.