A Senegalese patrol boat intercepted the vessel, suspected of involvement in the trade in which drug smugglers ship tonnes of cocaine from South America to Europe, using West African countries as transit points.
"During the search, the marine commando response team on board noticed that the ship's valves had opened," the armed forces said in a statement.
"Everything suggests that this sabotage action, consisting of sinking the ship, aimed to erase all evidence of the illicit cargo."
The patrol boat launched a rescue operation, saving seven of its own team and 10 members of the intercepted boat's crew, but a search is still underway for five missing marine commandos, it said.
(Reporting by Diadie Ba; Writing by Alessandra Prentice; Editing by Giles Elgood)
