Germany's opposition leader has vowed to bar people from entering the country without proper papers and to step up deportations if he is elected as chancellor next month, as a knife attack by a rejected asylum-seeker spills over into an election campaign in which he is the front-runner.

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.

German Chancellor Olaf Scholz said Wednesday he would demand to know why the Afghan suspect in a deadly knife attack had been able to stay in the country. "The authorities must explain as quickly as possible why the attacker was even still in Germany,

A deadly knife attack on a play school group in Germany, where an Afghan man was arrested at the scene, reignited a bitter immigration debate on Thursday, a month before elections.

The conservative opposition leader tipped to lead Germany following next month's elections has promised far-reaching changes to border and asylum rules after a group of children were targeted in a deadly knife attack in Bavaria.