The arrests were made in several cities in the kingdom, taking the total arrests linked to the double-murder to 18, said Abdelhak Khiam, head of Morocco's central office for judicial investigation.

"The two victims were stabbed, had their throats slit and were then beheaded," he told AFP.

The four main suspects in what the authorities describe as a terrorist act were arrested between Monday and Thursday last week in the tourist hub city of Marrakesh.

Danish student Louisa Vesterager Jespersen, 24, and 28-year-old Norwegian Maren Ueland were found dead at an isolated hiking spot south of Marrakesh on December 17.

Rabat's prosecutor has said the four main suspects pledged allegiance to the Islamic State group in a video filmed a week before the double-murder.

The killings have shaken Norway, Denmark and Morocco. A video circulated on social networks allegedly showed the murder of one of the tourists.

Morocco, which relies heavily on tourism income, suffered a jihadist attack in 2011, when a bomb blast at a cafe in Marrakesh's famed Jamaa El Fna Square killed 17 people, mostly European tourists.

An attack in the North African state's financial capital Casablanca killed 33 people in 2003.