U.S. authorities have seized a submarine-like vessel loaded with more than seven tonnes of cocaine off the coast of El Salvador, the Guardian reports.

The semi-submersible vessel was tracked in international waters off the El Salvador coast by U.S. aircraft. It was intercepted by the Coast Guard after a speedboat began to approach.

Inside the semi-submersible, authorities found 274 bales packed with 7,650kg (16,870 pounds) of cocaine worth hundreds of millions of dollars. Four suspected smugglers were taken into custody.

Traffickers use submarine-like vessels to move large amounts of cocaine because their low waterline profile makes them difficult to detect.

The interdiction is part of a 15-nation effort targeting international drug traffickers.