BRUSSELS—NATO will establish command centers in six of its eastern countries in coming months, Secretary-General Jens Stoltenberg said Friday, in part of a beefed-up response to Russian aggressiveness.

The outposts will form a chain of potential command centers for the North Atlantic Treaty Organization’s already announced new rapid-response force, which will consist of roughly 5,000 troops. Details are to be finalized at a meeting next week of NATO defense ministers.

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