Russia's raid on a ship just off Turkey's coast brings the fallout from the Ukraine war to another NATO frontier and raises the stakes as Ankara seeks to coax Moscow back to a grain-export deal that would restore some calm to the Black Sea.

Armed marines raided the Turkish-based vessel via helicopter on Sunday some 60 kilometers off Turkey's northwest coast, in international waters but near Istanbul, in what Moscow called an inspection before it sailed on to Ukraine.

Turkey, NATO's second-largest military, has made no public comment on the incident that occurred far south of the war that has raged for a year-and-a-half in the north Black Sea.