The Yugoslav Navy had 10,000 sailors (including 4,400 conscripts and 900 marines). This was essentially a coastal defence force with the mission of preventing enemy amphibious landings along the country's rugged 4,000-kilometer shoreline and coastal islands, and contesting an enemy blockade or control of the strategic Strait of Otranto. The entire coast of Yugoslavia was part of the naval region headquartered at Split. The naval region was divided into three smaller naval districts and a riverine flotilla with major naval bases located at Split, Šibenik, Pula, Ploče and Kotor on the Adriatic Sea, and Novi Sad on the River Danube. The strategic islands of Vis and Lastovo were heavily fortified and unauthorised entry was prohibited. The fleet was organized into missile, torpedo, and patrol boat brigades, a submarine division, and minesweeper flotillas. The naval order of battle included four frigates, three corvettes, five patrol submarines, fifty-eight missile, torpedo, and patrol boats, and twenty-eight minesweepers. One antisubmarine warfare helicopter squadron was based at Split on the Adriatic coast. It employed Soviet Ka-25, Ka-28, and Mi-14 helicopters, and domestic Partisan helicopters. Some air force fighter and reconnaissance squadrons supported naval operations.