Opposition head Friedrich Merz is calling for reforms to Germany's migration laws after a deadly knife attack in which an Afghan man facing deportation is the main suspect - even if they pass with support from the far-right Alternative for Germany (AfD).

The co-leader of Germany's Greens has asked for "clarification" from Friedrich Merz after hints that the conservative leader could turn to the far right for support on passing migration reforms after a deadly knife attack.

Germany's main opposition leader vowed on Thursday to impose immediate border controls if he becomes chancellor, as expected, after elections next month, a day after an Afghan asylum seeker was arrested for a deadly knife attack targeting children.