Police say a leftist group is claiming responsibility for an arson attack on Berlin's subway system that is causing delays for thousands of commuters.

Police spokesman Klaus Schubert said Friday an anonymous online message from the group claims it set fire to a power-cable shaft in the capital's Treptow neighborhood Thursday morning to call attention to the conditions of illegal refugees in Germany.

Schubert says police are trying to verify the authorship of the statement as part of the investigation into the attack.

Germany's national railway Deutsche Bahn, which operates the Berlin commuter system and whose trains use the same tracks, said repair works would last until Sunday. In the meantime, passengers are being transferred to buses.