German prosecutors say they have detained a 40-year-old man they believe may have been involved in last week's truck attack in Berlin.

Police said the Tunisian national was picked up after his phone number was found saved on the phone of Berlin attacker Anis Amri.

The man was detained as officials searched his home and business.

Prosecutors believe Amri was responsible for driving a truck into a Christmas market on December 19th, killing 12 people.

Officials have until Thursday evening to decide whether to request an arrest warrant, which will allow police to keep the man in custody pending possible charges.

The arrest comes as police investigate whether Amri travelled to the Netherlands immediately after the attack.

Sources close to the investigation told AFP news agency the 24-year-old boarded an overnight bus in Nijmegen, near the German border, that took him to Lyon in central France. 

CCTV cameras filmed Amri at Lyon-Part-Dieu rail station - where he travelled to Chambery in France.

Amri then travelled by train to Milan, northern Italy, where he was killed in a shootout with police.