The NATO reaction force currently numbers around 40,000 soldiers, who can deploy quickly when needed.
Coupled with other measures including the deployment of forces to defend specific allies, Stoltenberg said on Monday that the move to expand NATO forces is part of the "biggest overhaul of collective defence and deterrence since the Cold War".
He made the remarks at a news conference ahead of a NATO summit in Madrid, Spain on Tuesday, when the 30 military allies in the bloc are expected to also agree on further support to Ukraine after its invasion by Russia.