Putin unveiled an array of new nuclear weapons - that he said were either ready or in development - last Thursday, saying they could hit almost any point in the world and not be intercepted.
"We are of course concerned," Stoltenberg said during a visit to Iraq. "Because NATO does not want a new Cold War. We don’t want a new arms race."
Putin’s speech, weeks before an election he is expected to win, was in line with a pattern of behavior whereby Russia has developed new weapons and used military force on neighboring Ukraine, Stoltenberg said.
"This pattern which we have seen over some years is part of why NATO now is adapting and implementing the strongest reinforcement to our collective defense since the Cold War," he said.
"We’re not mirroring what Russia does - missile for missile, weapon for weapon - but we are responding because we see a more assertive Russia."
Stoltenberg renewed his call for Russia to abide by the Intermediate-Range Nuclear Forces (INF) Treaty that the United States said Putin had breached by his announcement last week.