Germany on Monday said it saw the passport-free Schengen zone "in danger" after Sweden and Denmark announced new controls at their borders to stem a migrant influx.

"Freedom of movement is an important principle -- one of the biggest achievements (in the European Union) in recent years," foreign ministry spokesman Martin Schaefer told reporters.

"Schengen is very important but it is in danger," he added, when asked about Denmark's announcement of checks at the German border, on the same day Sweden introduced controls on its frontier with Denmark.