By Nora Buli and Nina Chestney OSLO/LONDON (Reuters) – Russia maintained gas flows through key pipeline routes into Europe on Monday, despite uncertainty over payment terms and as European leaders urged further sanctions against Moscow amid allegations of war crimes in Ukraine. Physical gas flows through the Yamal-Europe pipeline, at Germany’s Mallnow border point see-sawed over the weekend and last stood at zero, data from operator Gascade showed. Nominations, or requests, for Russian gas deliveries via Slovakia’s Velke Kapusany entry point from Ukraine were steady on Monday at 967,954 MWh/da…