Two short-range ballistic missiles were launched on Wednesday in North Korea's second such test in less than a week.
New satellite images show Iskander ballistic missile units conducting drills within range of the border.
The U.S.-led NATO Western military alliance and Russia accused each other of fueling the Cold War–like crisis across Europe.
The new rocket is one of several developed for a short-range ballistic missile system that replaces nuclear-tipped arms, which are prohibited by an international treaty.
Poland has long called for U.S. backing in deterring Moscow, and Russian missiles are set to arrive near its border.
Russia's decision last year to deploy of the Iskander-M to a region bordering Poland and Lithuania prompted concern among both NATO allies.
Iskander units were drafted in the exercise close to the Estonian border.