German Chancellor Angela Merkel said on Tuesday that authorities believe a deadly rampage by a lorry driver at a Berlin Christmas market was a terrorist attack likely committed by an asylum seeker.

"According to what we know, we have to assume this was a terrorist attack," Merkel, visibly moved and dressed in black, told reporters.

"I know it will be especially hard for us to take if it is confirmed that the person who committed this attack sought protection in Germany."