Support for Germany's anti-migration AfD party soared to a year high of more than 15 percent in the wake of the Berlin truck attack, a poll to be released Saturday indicated.

With a general election expected next September, the right-wing populist Alternative for Germany recorded a 2.5-point boost to 15.5 percent compared to last week, according to the survey for the Bild newspaper by the Insa institute.

Chancellor Angela Merkel's Christian Democrats lost 1.5 points to hit 31.5 percent while the Social Democrats, junior partners in the ruling coalition, ceded one point to 20.5 percent.

The poll was carried out between Wednesday and Friday among 2083 eligible voters.

The prime suspect in the assault on the Berlin Christmas market which killed 12 people was a failed Tunisian asylum seeker.

The suspect, Anis Amri, who had been under surveillance as recently as September for suspected ties to the jihadist scene, was shot dead by Italian police in a gunfight in Milan early Friday.

In the immediate aftermath of the Berlin attack, the AfD blamed Merkel's liberal border policy, under which more than one million asylum seekers entered the country since 2015, for posing a serious security threat to the country.

One party official, Marcus Pretzell, even called the victims "Merkel's dead", in a tweet later criticised by the AfD leadership.

At a news conference on Friday Ms Merkel pledged a "comprehensive" analysis of what went wrong, and rapid reforms.

However she stressed that Germany would not sacrifice its principles in the fight against terrorism.

"Our democracy, our rule of law, our values, our humanity -- they are the alternative to the hateful world of terrorism, and they will be stronger than terrorism," she said.

Ms Merkel also said the incident raised many questions and her government would take measures to improve security.

"We have also made progress this year on the very important issue of deporting Tunisian citizens who have no right to stay in Germany," she said.

"I told the Tunisian president that we have to significantly speed up the deportation process and increase the number of people sent back."