The European Union said Friday it was concerned about the impact on competition of German airline Lufthansa's plan to take over routes operated by failed rival Air Berlin.

"We have quite deep competition concerns because there is a risk that on some routes Lufthansa becomes de facto a monopoly," EU competition commissioner Margrethe Vestager said at a news conference.

The warning comes a week after Lufthansa offered concessions to encourage the European Commission to green-light its planned purchase of parts of Air Berlin.

The Frankfurt-based airline group hopes to pay 210 million euros ($250 million) for 81 aircraft from Air Berlin's 140-strong fleet, plus takeoff and landing slots, as well as Austrian subsidiary Niki.