Two U.S. Navy warships on Monday sailed through the Bosporus Strait into the Black Sea _ waters dominated by the Soviet fleet, an Istanbul port source said. The vessels were the destroyer Thorn and frigate Capodanno, both attached to the Mediterranean-based 6th Fleet, said the source, speaking on condition of anonymity. The Ocean, a Soviet navy intelligence and support ship, also passed through the Bosporus into the Black Sea together with the U.S. warships, the source said. The U.S. ships will spend four days in the international zones of the Black Sea, the source said. The United States dispatches ships to the area from time to time to reaffirm its commitment to the vital Turkish straits and place a stake in navigational rights in the Black Sea.