Jens Stoltenberg urged Russian Federation to return to compliance with Intermediate-Range Nuclear Forces (INF) treaty it agreed bilaterally with the United States in 1987.
As Stoltenberg said, the North Atlantic Treaty Organisation defense ministers will discuss Russia's deployment of "several battalions of the SSC-8 [9M729] missile".
Russian Federation suspended the INF treaty earlier this month after Washington announced it would withdraw in six months unless Russian Federation ended what it says were violations of the pact.
The INF, which ended a buildup of warheads in Europe, bans the production and deployment of land-based cruise and ballistic missiles with a range of 500-5,500 kilometers.
The two-day meeting in Brussels is the first chance for North Atlantic Treaty Organisation ministers to debate what steps the alliance will take to bolster its defence against new Russian medium-range missiles.
"At this meeting of defense ministers, we will discuss what steps North Atlantic Treaty Organisation should take to adapt to a world with more Russian missiles", Stoltenberg asserted. "We all know that a treaty that is only respected by one side can not keep us safe".
The Pentagon believes that Russia's ground-fired cruise missile could give Moscow the ability to launch a nuclear strike in Europe with little or no notice.
Russian President Vladimir Putin responded to the USA withdrawal from the treaty by announcing it would also pull out.
While Washington and Moscow are at odds over the INF, the treaty does nothing to constrain China, whose fast-growing military depends on medium-range missiles as a key aspect of its defense strategy.
The U.S. doesn't have immediate plans to deploy new missiles to Europe when the withdrawal takes effect in August, according to two administration officials involved in the deliberations who briefed reporters last week on condition of anonymity.
The U.S. ambassador to NATO, Kay Bailey Hutchison, sought to reassure them.
The top US envoy to the North Atlantic Treaty Organization said Washington was open to a broad treaty with Russian Federation to curb the proliferation of nuclear weapons while also warning Turkey not to purchase a new arms system from Moscow.
President Recep Tayyip Erdogan has been adamant that Turkey would take the Russian missile system, saying traditional allies in the West failed to meet his country's defensive needs.