BERLIN - "We must assume it was a terrorist attack," Germany Chancellor Angela Merkel said here Tuesday, referring to an attack at a Christmas market in Berlin on Monday night that killed 12 people and injuring many others.

"I am thinking of the dead and the injured, the relatives and the friends, a whole country is unified in their grief," she said, adding that the act will be punished "as hard as our laws require."

Together with Berlin's mayor, Merkel will visit the accident site on Breitscheidplatz square near the iconic Kaiser Wilhelm Memorial Church to express her sympathy to the victims.

Merkel also noted that she regrets the fact that the suspected person could be a refugee.

According to Berlin police, it is suspected that the truck used in the attack was stolen from a construction site in Poland and was deliberately driven into the crowd at the Christmas market on Breitscheidplatz square.

They said a Polish man was found dead inside the vehicle but added he was not in control of it.