TUNIS (AFP) - Tunisian anti-terrorism police were on Wednesday (Dec 21) questioning the family of Anis Amri, the prime suspect in the deadly truck assault on a Berlin Christmas market, a security official told AFP.

"A unit of the anti-terrorism brigade has questioned the suspect's family," the official said, speaking on condition of anonymity.

German prosecutors on Wednesday named 24-year-old Tunisian national Amri as their main suspect in the attack claimed by the Islamic State in Iraq and Syria (ISIS) group, which killed 12 people.

The Tunisian security official said Amri's parents, who live in the central town of Oueslatia, were being questioned.

Amri has four sisters and a brother, the source said, but it was unclear if anyone else was being questioned.

The security source said Amri had been arrested several times in Tunisia for alleged drug use.

He fled Tunisia to Italy after the 2011 revolution that overthrew longtime dictator Zine El Abidine Ben Ali and spent three years there before travelling on to Germany, the source said.

"When I saw the picture of my brother in the media, I couldn't believe my eyes. I'm in shock, and can't believe it's him who committed this crime," his brother Abdelkader Amri told AFP.

But "if he's guilty, he deserves every condemnation. We reject terrorism and terrorists - we have no dealings with terrorists."

His sister Najoua said: "I was the first to see his picture and it came as a total shock. I can't believe my brother could do such a thing.

"He never made us feel there was anything wrong. We were in touch through Facebook and he was always smiling and cheerful," she added.

Contacted by AFP, Tunisia's interior and foreign ministries refused to comment on the case.

Upon naming Amri as the prime suspect, the federal prosecutor's office in Germany offered a reward of 100,000 euros (S$150,000) for information leading to his arrest.

It also warned he "could be violent and armed".