NATO allies will boost high readiness forces to "well over 300,000", alliance chief Jens Stoltenberg said Monday.

Leaders from the military alliance will meet in Madrid this week for what Stoltenberg said would be a "transformative" summit.

Stoltenberg said allies would bolster some of their battle group formations along NATO's eastern flank "up to brigade level", tactical units of some 3,000-5,000 troops, and ratchet up high readiness numbers to "well over 300,000".

In addition, more heavy weaponry including air defense systems would be shifted forwards, and forces pre-assigned to defend specific NATO members on the alliance's exposed eastern edge.

Stoltenberg did not give further details about the additional high readiness forces or how they could be deployed by the alliance.

NATO currently has a high readiness force of around 40,000 troops under its command.

The more than 300,000 troops are expected to form a larger pool that the alliance could tap into in the case of an emergency.

NATO now has eight battle groups across its eastern members and Stoltenberg said some of these, likely in the Baltics and Poland, would be bolstered to "brigade level".