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.

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.

A two-year-old boy and a 41-year-old man were killed on Wednesday in a knife attack in Germany's Aschaffenburg. A 28-year-old man from Afghanistan has been arrested by the police.