MONS, Belgium—Senior military officers from the North Atlantic Treaty Organization described Russian military forces deployed in more than 100 makeshift bases just across the Ukraine border as being in a state of high readiness—able to move swiftly within hours of a command from the highest level.

In a rare briefing to reporters at the alliance's military headquarters in southern Belgium that officials said was aimed at countering "misperceptions" promoted by Russia, NATO officers showed for the first time commercial...