BERLIN — German authorities are investigating a string of small arson attacks on railway facilities across the country that came as the country prepares to host next month’s Group of 20 summit.

Federal police said that there were incidents Monday morning in Berlin, Hamburg, Cologne, Dortmund, Leipzig, Bremen and Bad Bevensen. The fires, laid in cable ducts beside the tracks and elsewhere, disrupted trains in some locations.

Interior Ministry spokesman Tobias Plate said there were “13 cable fires caused by unconventional explosive and incendiary devices” in total.

Far-left activists have threatened attacks ahead of the G-20 summit of major world leaders that Germany is hosting in Hamburg July 7-8. Plate said authorities are investigating the fires but “it is too early to confirm such connections reliably.”