At talks between Russian and NATO ambassadors on Wednesday, Stoltenberg said, "We regret that Russia has not heeded our calls" for transparency about the missile system.
NATO says Russia's 9M729 cruise missile contravenes the 1987 Intermediate-Range Nuclear Forces Treaty. The pact between the U.S. and the Soviet Union banned land-based cruise and ballistic missiles with ranges between 500 and 5,500 kilometers (310-3,410 miles.)
Stoltenberg says Russia's reluctance to discuss it reinforces NATO's belief that the system "poses a serious risk to the strategic stability of the Euro-Atlantic area."
Russia says the missiles respect the treaty. President Donald Trump has threatened to abandon the pact.
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