NATO says it is concerned about a Russian missile system that could carry nuclear warheads, and believes it violates a landmark Cold War arms treaty.
The US-led military alliance said in a statement Friday that "allies have identified a Russian missile system that raises serious concerns."
It urged Russia "to address these concerns in a substantial and transparent way, and actively engage in a technical dialogue with the United States."
NATO fears the system, which it did not identify, contravenes the 1987 Intermediate-range Nuclear Forces Treaty.
The Cold War-era pact bans an entire class of weapons -- all land-based cruise missiles with a range between 500 and 5,500 kilometers (310-3,410 miles).