A total of 6,500 personnel and 38 warships from 44 navies are taking part in a U.S.-led mine countermeasures exercise in the Middle East, officials said Sunday.

The international exercise, the third and largest so far, is being conducted by the U.S. 5th Fleet, whose area of responsibility covers about 2.5 million square miles of water. Its sea phase started on Sunday, although preparations have been ongoing for several days.

The Persian Gulf, Gulf of Oman, Gulf of Aden and Red Sea contain three critical choke points for maritime shipping and navigation. Nearly 20 percent of the world’s oil supply transits through the Strait of Hormuz.