Populists across Europe have seized on the truck attack in Berlin as a way to criticise Germany's immigration policy but key players have held back on jumping to conclusions as the investigation continues.

Former UK Independence Party (UKIP) leader Nigel Farage, a key ally of US President-elect Donald Trump in Europe, said the attack which killed 12 people was "no surprise" and would be part of Merkel's "legacy".

"Merkel has directly caused a whole number of social and terrorist problems in Germany, it's about time we confronted that truth," he told LBC radio on Tuesday.

UKIP donor Arron Banks, who was also a key funder behind the Brexit campaign, tweeted that Merkel "might as well have" been driving the truck herself.

A 23-year-old Pakistani asylum-seeker was arrested immediately after the incident on Monday after reportedly fleeing the scene but was released on Tuesday without charge.

Police said on Wednesday they were now on a manhunt for a new suspect, identified in German media as a Tunisian citizen in his early 20s who applied for asylum in April and had a temporary residence permit.

Merkel has been criticised over her decision to let in around a million migrants -- many of them fleeing war-torn Syria -- over the past two years.

- 'Last drop of patience' -

Her policy has been polarising, not just in Germany.

Just hours after Monday's attack, far-right Dutch lawmaker Geert Wilders sharply blamed European leaders for admitting asylum-seekers into Europe.

"Merkel, (Dutch Prime Minister Mark) Rutte and all the other cowardly government leaders have allowed in Islamic terror and an asylum tsunami with their open borders policy," he tweeted on Tuesday.

Wilders, who heads the anti-Islam Freedom Party (PVV), also tweeted a photo-shopped picture of Merkel with her hands, face and jacket spattered in blood.

The image was not accompanied by any words, but implied she had blood on her hands for the attack.

Slovak Prime Minister Robert Fico on Tuesday said the Berlin attack had been "the last drop in the cup of patience" in Europe's migration crisis.

Matthew Goodwin, a senior fellow at the Chatham House think tank, said attacks such as the one in Berlin represented a "significant opportunity" for the "radical right" to emphasise the issue of security.

"Across much of Europe, the radical right is increasingly linking the migrant crisis to security," alongside their traditional anti-elite and anti-immigration campaign messages, he said.

- Merkel 'irresponsible' -

Other populist forces have been more cautious, however.

In Austria, the far-right Freedom Party steered clear of making the link between Merkel's policies and the attack.

France's National Front leader Marine Le Pen also made no connection.

But the National Front's deputy leader Florian Philippot told French TV that Islamic State group militants had infiltrated Europe along with migrants.

"When there are Islamist terrorists who infiltrate themselves in a massive influx, we have the duty to stop the influx," he said, calling Merkel's open-doors policy for migrants "irresponsible".

In Germany itself, the Islamophobic and anti-immigration populist party AfD wasted no time in laying the blame on Merkel.

"The milieu in which such acts can flourish has been negligently and systematically imported over the past year and a half," the group's co-leader Frauke Petry said in a statement, in a clear reference to Merkel's decision to let in refugees.

"Germany is no longer safe. It should be the responsibility of the chancellor to tell you this. But since she won't do it, then I'll say it," Petry said, demanding "control over our territory, no ifs and buts".