German Chancellor Angela Merkel said she was "horrified, shaken and deeply saddened" on Tuesday about a truck attack that killed at least a dozen people at a Berlin Christmas market.

"I want you to know that all of us, a whole country, is joined with you in mourning and sadness," Merkel told reporters in the German capital.

A truck slammed into a crowded Christmas market in central Berlin on Monday night, killing at least 12 people and wounding 48, 18 of them seriously, German Interior Minister Thomas de Maiziere said at a press conference. Only a few of the victims have been identified so far, he said.

"There is much we still do not know with sufficient certainty but we must, as things stand now, assume it was a terrorist attack," Merkel said.

During his press conference De Maiziere said "there is no doubt anymore that the horrible event last night was an attack."

Suspect denies involvement

The main suspect in the attack is a 23-year-old Pakistani citizen who arrived in Germany on December 31, 2015 and had applied for asylum, the Interior Minister said. The man has denied any involvement in the attack.

Merkel said that all of Germany was united in grief, adding that it would be particularly painful if the attacker "came to Germany for protection and asylum."

Merkel promised that those responsible for the attack would be punished "with the full strength of our law."

She added that the suspected attack should not keep Germans from living their normal lives and visiting Christmas markets in the future.

"We do not want to live with the fear of evil paralyzing us," Merkel said.

A 'deliberate' act

Investigators in the German capital, Berlin, are assuming that a truck that plowed into a Christmas market "was deliberately steered into the crowd." 

The suspected driver was detained near the scene while a passenger was found dead in the truck. The dead passenger has been identified by police as a Polish national who appeared to have been shot by a pistol, de Maiziere said. The weapon has yet to be recovered, he added.

German special forces stormed a hangar at Berlin's Tempelhof airport where the city's largest refugee shelter is located, reported German newspaper "Die Welt." The suspect was allegedly registered at the refugee center there, the paper said.

The Interior Ministry said Germany's Christmas markets and other large events will continue to take place but with "adjusted" security measures - although Berlin's markets will remain closed on Tuesday out of respect for the victims.

Berlin police will hold a press conference in the afternoon.

Heightened security

The Scania-brand truck rammed up to 80 meters (260 feet) into the Christmas market near the iconic Kaiser Wilhelm Memorial Church at around 8:30 p.m. local time as locals and tourists gathered to enjoy the evening. Located along the Kurfürstendamm shopping mile, it is one of the most popular and well-attended out of Berlin's 50 holiday markets.

The truck had Polish license plates and was carrying steel beams. Earlier, the owner of the truck said he feared the vehicle may have been hijacked.

Authorities in Germany, France, the United Kingdom, Belgium and the Netherlands said they are now reviewing and heightening security measures at Christmas markets and other holiday events.

The likely attack, one of the biggest to hit Germany in more than a decade, comes as Europe is on high alert following a string of terrorist attacks in France and Belgium over the past year. Although no group has claimed responsibility for the incident in Berlin, the so-called "Islamic State" (IS) has previously called on its followers to carry out terror strikes in Europe.

Policies called into question

Following the suspected attack, some German politicians have called for changes in Germany's migration policy while others cautioned against inciting hatred against migrants. 

"We owe it to the victims, to those affected and to the whole population to rethink our immigration and security policy and to change it," said Horst Seehofer, the leader of Bavaria's Christian Social Union (CSU) - the sister party of Merkel's Christian Democrats (CDU).

Senior members of Germany's far-right, anti-immigrant Alternative for Germany (AfD) party harshly criticized Merkel's liberal migration policies. Senior AfD member Marcus Pretzell went so far as to blame Merkel for the attack on Twitter.

AfD leader Frauke Petry said Germany was no longer safe and that "radical Islamic terrorism has struck in the heart of Germany."

Germany's Green party, on the other hand, warned that the AfD was using the attack in order to rile up anti-foreigner sentiments.

"We don't need any more incidents like these," Green party head Cem Özdemir told German public broadcaster ZDF, referencing Pretzell's tweet.

"They're debunking their own message," he added.

rs/tj (AP, AFP, dpa, Reuters)