A group of Colombian drug traffickers sank their submarine off the coast of Spain after they were spotted by customs officers, police said in a joint statement Tuesday.

Spain is one of the main entry points for drugs into Europe, due to its close links with Latin America and its proximity to Morocco.

On Sunday, customs officers spotted a vessel 280 nautical miles (500 kilometres) off the southern Spanish coast.

It was a 20-metre semi-submersible -- a type of boat increasingly used by traffickers who are redoubling their creativity to find new ways of concealing drug shipments. 

When customs officers approached, the crew "opened the valves on the bottom of the submarine, before surfacing to safety," the statement said.

It added: "The semi-submersible was flooded within minutes, before sinking with its cargo to the bottom of the sea."

The four Colombian crew members were rescued and then arrested.