SIX more suspects have been held in Morocco as cops probe terror links of a gang of ISIS thugs who beheaded two Scandinavian tourists.

The arrests bring the total to 19 people now in custody over the murders of Louisa Vesterager Jespersen, 24, from Denmark, and her university pal Maren Ueland, 28, of Norway.

They were butchered at their campsite in the Atlas mountains on December 17 - then video of the slaughter was sent to their families and spread on Facebook.

Authorities in Morocco have now rounded up a total of 15 people linked to the four ISIS fanatics accused of the murders.

They include nine suspects held before Christmas as authorities claimed they had foiled another terror plot and found bomb-making materials.

The four suspected killers pledged allegiance to jihadi "Caliphate" Abu Bakr al-Baghdadi in a video that emerged after they were captured last week.

But officials described them as a "lone wolf" gang with no formal ties to ISIS.

Boubker Sabik, spokesman for Morocco's security agency, said the victims were not selected in advance and it was not directed by ISIS.

He said: "The crime was not coordinated with Islamic State. Lone wolves do not need permission from their leader."

One of the thugs was a known extremist who was arrested in 2013 for trying to join jihadis abroad, according to reports.

It is alleged he radicalised the other three.

They made their ISIS video - posing in front of a black jihadi flag - three days before Louisa and Maren's bodies were found.

It is believed they may have followed the women from Marrakech to Imlil as they set off for a trek at Mount Toubkal, North Africa's highest peak.

Witnesses saw men striking their own camp around 600 yards from the backpackers' tent.

They were attacked as they slept at night. Sickening mobile phone footage shows one victim screaming as a fanatic hacks at her throat with a large blade.

Men can be heard shouting "It's Allah's will" and "These are your heads, enemies of God", and also claiming revenge for the deaths of jihadi "brothers" in Syria.

The video provoked outrage in Morocco as it spread on Facebook and other social platforms.

ISIS sympathisers also sent clips and images to the victims' families.

It was also claimed the depraved killers sent the video directly to the victims' friends online.

Horrified Moroccans have demanded they face the death penalty.

Morocco is said to be beefing up its security efforts to combat ISIS fighters returning from Syria and Iraq.

Authorities said 242 of the 1,669 Moroccans who joined the terror group had been arrested.

Some had used false passports and tried to hide among refugees heading for Europe as ISIS began to lose key battles in the Middle East.