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.

Friedrich Merz, the conservative front-runner in the race for Germany’s chancellorship, called for a radical overhaul of the country’s migration policy including permanent border checks. The comments come after an asylum seeker from Afghanistan was taken into custody over a fatal stabbing of a two-year-old child and a man in a park on Wednesday.