KEHL, Germany — Germany on Monday began random checks at its borders with five Western European nations as it seeks to crack down on irregular migration, expanding a system of mobile border controls that are already in place at four other borders.

The checks began at the borders with France, the Netherlands, Belgium, Luxembourg and Denmark before dawn, and are initially scheduled for six months. Germany has already been carrying out the checks at its borders with Poland, the Czech Republic, Austria and Switzerland since last year.