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.

Police identified the suspect as an Afghan man. Politicians gearing up for an election responded with comments about migration and law and order.

German police say two people, including a two-year-old boy, were killed and two others were severely injured in a stabbing attack in Bavaria.