MOSCOW, Sept. 15 (Xinhua) -- Russia's Black Sea Fleet ships are constantly monitoring the U.S. navy ship USNS Yuma, which passed through the Bosporus Strait on Saturday evening, the Ministry of Defense said Sunday.

The purpose of the U.S. expeditionary fast transport vessel is unknown, and it is the third time the ship entered the Black Sea this year, the ministry's Zvezda broadcasting service said.

According to it, last time, the USNS Yuma visited the Ukrainian port of Odessa to participate in the multinational maritime exercise Sea Breeze 2019 in July, which was co-hosted by Ukraine and the United States.