NATO's chief says the military alliance would be concerned if reports that Russia has violated a Cold War-era treaty by deploying a cruise missile prove true.

U.S. intelligence agencies have assessed that the missile became operational late last year, possibly violating the 1987 Intermediate-Range Nuclear Forces Treaty on the development and testing of cruise missiles.

NATO Secretary General Jens Stoltenberg said Wednesday that "compliance with arms control agreements is of great importance and especially when it comes to treaties covering nuclear weapons."

He said that "any non-compliance of Russia with the INF Treaty would be a serious concern for the alliance."

His remarks in Brussels came before chairing the first meeting of NATO defense ministers with U.S. Defense Secretary Jim Mattis.