Tunisian police have arrested the nephew of the suspected Berlin Christmas market attacker and two other men thought to be connected to Anis Amri.

A statement says the three suspects were members of a "terrorist cell" that was "connected to the terrorist Anis Amri who carried out the terrorist attack in Berlin".

This trio, according to Tunisia's Interior Ministry, includes 18-year-old nephew "Fredi" who Amri allegedly sent money to so he could join him in Germany.

Italian police and forensics experts surround Anis Amri's body in Milan in the early hours of Friday morning Daniele Bennati/AFP/Getty (Daniele Bennati/AFP/Getty)

"One of the members of the cell is the son of the sister of the terrorist and during the investigation he admitted that he was in contact with his uncle through telegram," read the statement.

Amri allegedly urged his nephew to adopt jihadist 'takfiri' ideology and "asked him to pledge allegiance to Daesh".

The nephew also reportedly said that his uncle was the "prince" of a jihadist group based in Germany known as the "Abu al-Walaa" brigade.

Video surfaces of Berlin attack suspect Anis Amri

The 24-year-old was shot dead by police in Milan in northern Italy after Amri opened fire when he was challenged during a routine patrol at around 3am local time.

The Tunisian, whose father said he was a drug-taking troublemaker who became a radical after moving to Europe, was traced across the continent after being linked to the bus crash that killed 12 people and injured 48.

Amri, who reportedly shouted 'Alluahu Akbar' in the Milan shootout, was hunted after his fingerprints and wallet were allegedly found in the truck.

Mustapha and Nour-Houda Amri, the parents of 24-year-old Anis Amri Fethi Belaid/AFP/Gettty (Fethi Belaid/AFP/Gettty)

Police in Spain are also investigating whether Amri was in contact with an extremist in Spain following a tip-off from German authorities, according to the Associated Press.

Spain's Interior Minister, Juan Ignacio Zoido, told Spanish radio station Cope: "We are studying all possible connections [between Amri] and our country, above all with one specific person."

Amri's nephew was reportedly arrested in his hometown of Oueslatia while the other two were arrested in Tunis - reports the Associated Press.