Spanish police said Tuesday they had broken up an international drug-smuggling ring, seizing four tonnes of cocaine and arresting 28 people. 

The gang, which was made up of "experienced sailors and known traffickers", was working with major national and international drug smugglers to "bring in cocaine shipments by sea", a police statement said. 

Police said it was the biggest drug-smuggling organisation in the northwestern region of Galicia, and had "a large number of boats and speedboats" stowed away at different locations between northern Portugal and the port city of Pontevedra.

With support from the US Drug Enforcement Agency, Britain's National Crime Agency and the Colombian police, Spanish investigators discovered that the traffickers were poised to receive a shipment and started looking for "the mothership" which they intercepted some 200 nautical miles off the coast. 

On board the Togo-flagged vessel, which had been at sea for nearly three weeks, they found the cocaine and arrested its 15-strong crew, most of whom were Bangladeshi or Nepalese. 

At the same time, police raided 20 addresses in Galicia, arresting another 13 suspects, some of whom has previous convictions for drug smuggling. 

Galicia's rugged Atlantic coast with hundreds of hidden coves, inlets and desolate beaches has for years been the gateway for illegal drugs to reach Europe.

In November, police intercepted a 20-metre submarine off the Galician coast which was carrying three tonnes of cocaine worth €100 million, the first time a "narco submarine" had been found in Europe.