NATO ignored protests from Ankara and sent warships steaming into the Black Sea on Thursday for what the military alliance insisted were "long-planned exercises" and "routine visits" to ports in Romania and Bulgaria.
Three warships from Spain, Germany and Poland are due to be joined by US frigate the USS Taylor on Friday.
Turkey opposes the deployment of NATO navy forces on the Black Sea, saying that the region is safe. It points out that the NATO deployment breaches the 1936 Montreux Convention, which limits the total weight of the warships that a country that does not border the Black Sea can deploy to 45,000 tons.
NATO officials claimed that the deployment was not linked to tensions over Russia's military intervention in Georgia, which lies on the sea's eastern shore. But it risks increasing tensions with Moscow, which has sent ships from its Black Sea fleet to the Georgian coast.