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NATO's secretary-general has warned that Russia's violation of a key Cold War-era treaty is one of the most pressing security challenges for the alliance.

Jens Stoltenberg urged Russia to return to compliance with the Intermediate-Range Nuclear Forces (INF) treaty, which has been the cornerstone for arm control for decades.

Stoltenberg said Friday after talks with Bulgarian Prime Minister Boyko Borissov in Sofia that "NATO does not want a new cold war, we don't want a new arms race and we call on Russia to come back into compliance with the INF Treaty."

He added that at the same time NATO needs to be prepared for a world without the INF Treaty and with more Russian missiles.

Stoltenberg was in Bulgaria for the 15th anniversary of the country joining NATO.