Cross-border German train services running from Poland will be free of charge for Ukrainians fleeing the war in their country, Germany's state-owned rail operator Deutsche Bahn (DB) announced Sunday.

Passengers wishing to make use of the option should present a Ukrainian passport or identity card, DB said.

"This will ease onward travel from the border for those fleeing," it said, adding that the measure had already been put in place.

DB said it was working in close collaboration with rail authorities in Poland, the Czech Republic and Austria, all countries through which Ukrainian refugees might pass.

It said it was also preparing for additional capacity in cooperation with Poland's rail operator in the form of additional carriages or special trains.

Up to six long-distance trains enter Germany from Poland daily, all crossing the border at Frankfurt an der Oder.