FILE PHOTO: German Foreign Minister Heiko Maas addresses an event discussing issue of '100 years of German Polish policy' in Berlin, Germany, November 15, 2018. REUTERS/Fabrizio Bensch

BERLIN (Reuters) - German Foreign Minister Heiko Maas on Tuesday urged Moscow to stick to international law again and respect the territorial sovereignty of neighboring countries, following Russia’s seizure of Ukrainian vessels.

Ukraine’s President Petro Poroshenko has warned of an “extremely serious” threat of a land invasion after Russia seized three Ukrainian naval ships and took their crew prisoner at the weekend.

“The aim must be that Russia sticks to international rules again and that it does not violate the territorial sovereignty of its neighbors,” Maas said in a speech in Berlin.

To achieve this, Germany and its European allies need clear principles and a “true dialogue” on common security in Europe, Maas added.