Germany's opposition leader Friedrich Merz is under fire for vowing strict border controls if he is elected chancellor, with the frontrunner citing a deadly knife attack that was allegedly carried out by a rejected asylum seeker as justification for a migration overhaul.

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