German police intensified a manhunt for the driver of a truck, who killed 12 people when he drove through a crowded Berlin Christmas market at high speed, while concerns about the political and social fallout mounted.

After releasing a Pakistani asylum-seeker arrested near the scene, authorities warned on Wednesday the attacker is on the run and may be armed. They also said it was unclear if the perpetrator was acting alone or with others.

The 25-tonne truck smashed into wooden huts serving mulled wine and sausages, injuring about 45 people, many seriously. Six of those killed were Germans and the Polish driver of the truck was found shot dead in the cabin of the vehicle.

The Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group has claimed responsibility for the attack.

German police were planning an "imminent" operation in the western state of North Rhine-Westphalia related to the truck attack, security sources told the German press agency dpa on Wednesday.

The information came shortly after media reports said police were hunting for a Tunisian suspect in his 20s whose immigration papers were found inside the truck. 

ISIL makes Berlin attack claim amid hunt for suspects

The head of the Association of German Criminal Detectives told German television late on Tuesday police hoped to make news arrests soon.

"I am relatively confident that we will, perhaps tomorrow - or in the near future - be able to present a new suspect," Andre Schulz told a talk show on the ZDF public channel.

Wednesday's Passauer Neue Presse newspaper quoted the head of the group of interior ministers from Germany's 16 federal states, Klaus Bouillon, as saying tougher security measures were needed.

"We want to raise the police presence and strengthen the protection of Christmas markets. We will have more patrols. Officers will have machine guns. We want to make access to markets more difficult, with vehicles parked across them," Bouillon told the newspaper.

German Chancellor Angela Merkel, who will run for a fourth term next year, said it would be particularly repulsive if a refugee, seeking protection in Germany, was the perpetrator.

Some politicians have blamed her open-door migrant policy for making such attacks more likely.

The anti-immigrant Alternative for Germany (AfD), which has won support in the last two years as Merkel's own popularity has waned, said on Tuesday that Germany is no longer safe.

Suspect in Berlin Christmas market attack released

Yascha Mounk, a lecturer at Harvard University and a Fellow at the Transatlantic Academy of the German Marshall Fund, told Al Jazeera Monday's deadly incident will have serious repercussions on the fabric of German society.

"After the attack, things are going to get worse for ethnic minorities within Germany and much worse for establishment parties and politicians," said Mounk. "I think we are now in a very dangerous political moment that will change the country."

He said Germans will associate foreigners with the attack, noting Merkel's popularity has plummeted since she allowed hundreds of thousands of refugees into the country over the past two years.

"The influx of refugees has already taken a huge toll on Mekel's popularity … She has paid dearly for her moral courage," Mounk said. "Without a doubt, we are going to see a real test to the German political system."

Backlash feared

The sole suspect, a 23-year-old Pakistani asylum-seeker, was released late on Tuesday for lack of evidence

ISIL's Amaq website said "a soldier of the Islamic State" carried out the Berlin carnage.

Europe has been on high alert for most of 2016 with attacks striking France and Belgium.

"We've all been prepared that something like this could happen, so we were not surprised," said economics student Maximilian Much, 24.

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The attack also raised concerns about a possible backlash.

"No question, the atmosphere in this country will change and become more tense," said Tarik Elsayed, a 22-year-old German-born son of Egyptian parents. "Of course, as an Arab, I will get more hostile looks, it will get only worse now."

But Tarek Elmasoudi, an Egyptian asylum-seeker, said he was not afraid. "The Germans are very nice and I want to stay here."

Source: Al Jazeera and news agencies