Spanish police say they have busted a drug-smuggling ring and arrested 20 people from four countries on suspicion they were using helicopters to bring hashish from Morocco into Spain.

It was later discovered that the pilot was serving a prison sentence and using weekend furloughs to fly in drugs hauls, a police statement says.

The investigation began when detectives found out the gang was planning a landing near southwestern Estepona. Agents moved in and arrested the pilot and others on March 6.

They later raided 14 houses in locations including Madrid, Malaga, Seville and Cadiz, arresting suspects of Spanish, Moroccan, Romanian and Ecuadorian nationalities, Saturday's statement says.

Two helicopters, a truck, 1,500 kilograms (1.7 tons) of hashish, a marijuana plantation and 220,000 euros ($245,695) were also seized, the statement says.