Italian and German anti-terrorist police met in Milan on Saturday to determine how Berlin terror suspect Anis Amri travelled 1000 miles and crossed the borders of at least three countries before he was shot dead, amid fears of revenge attacks from Islamist sympathisers in Italy.

The high-level meeting took place as Tunisia arrested Amri’s nephew and two others accused of being members of a “terrorist cell” that supported Amri in the attack that killed 12 people in the Christmas market.

A statement from the Tunisian interior ministry said the three suspects, aged between 18 and 27, were arrested on Friday and were members of a "terrorist cell... connected to the terrorist Anis Amri".

The statement made no direct link between the suspects and Monday's deadly attack in Berlin. The interior ministry said that Amri had sent money to his nephew and encouraged him to pledge allegiance to Isil.

"One of the members of the cell is the son of the sister of the terrorist (Amri) and during the investigation he admitted that he was in contact with his uncle through (the messaging service) Telegram," it said.

Amri allegedly urged his nephew to adopt jihadist "takfiri" ideology "and asked him to pledge allegiance to Daesh (Isil)," it said.