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.

The conservative leader wants to impose strict border controls after knife killings in Aschaffenburg this week.

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.