The Spanish National Police's Special Operations Group, or GEO, seized a ship carrying 3,000 kilos of high purity cocaine some 50 miles from the coast of the southern city of Cadiz, officials said Wednesday.

A total of 31 suspected members of a Spanish-based international drug-trafficking network were arrested in the operation, dubbed "Espartana" and launched eight months ago.

The drugs had been loaded off South America and were being shipped to the coast of the northwestern Spanish region of Galicia for later transport to Madrid, National Police director Ignacio Cosido said in a press conference.

Branches of the drug-trafficking network in Colombia, Bulgaria and Spain were investigated as part of the operation, Cosido said.

The National Police will conduct an inspection of the vessel - the SV Nikolay, which is docked at the Cadiz port - throughout the day on Wednesday.

The three branches probed in the operation are "very differentiated," Cosido said, adding that six Spain-based suspects affiliated with the Colombian branch were arrested.

Authorities said another 21 people detained on board the seized ship were linked to the Bulgarian branch, responsible for maritime shipments of the illicit drugs.

Four other detainees are suspected of belonging to the network's Spanish branch, based in Galicia and responsible for distributing the drugs throughout Spain and putting them at the disposal of the Colombians.

The Spanish police carried out the eight-month operation in cooperation with the security forces of Bulgaria, where more arrests could be made.

The cocaine was loaded somewhere in the Caribbean Sea, near the South American coast but outside a country's territorial waters. EFE