This image made from video released by Amaq News Agency of the Islamic State group shows Anis Amri, a Tunisian suspect in the Berlin truck attack pledging allegiance to its leader Abu Bakr al-Baghdadi and vowing to fight against what he calls "the Crusader pigs." The video, which appeared to have been taken by Anis Amri himself, shows him standing on a footbridge in the north of Berlin, not far from where he allegedly hijacked the truck used in the attack that killed 12 people and injured dozens more at a Christmas market. Photo: Militant video via AP

Berlin - Berlin Christmas market attacker Anis Amri spent a prolonged period of time in Switzerland and possibly even made several trips there, Germany's ZDF television reported on Friday.

The 24-year-old Tunisian drove a large lorry into revellers on December 19, leaving 12 dead. He was shot dead by Italian police on the outskirts of Milan four days later.

Not only was he able to travel from Germany to Italy after the attack but investigators are now checking to see how long he spent in Switzerland before the atrocity, ZDF said.

The station added that contact data, found on Amri's mobile phone following the Berlin attack, is being reviewed by Swiss authorities.

Swiss prosecutors opened a case related to the Berlin market attack on Wednesday, without specifying who is the focus of the proceedings.

The case involves suspicion of supporting a criminal group as well as a violation of the ban on membership of terrorist militias, such as Islamic State.

ZDF said investigators were particularly interested in whether Amri was in Switzerland when he obtained the gun - an Erma pistol - which he used to murder the Polish driver of the truck.

Some Swiss cities have a burgeoning Islamist scene.