Russia's oil pipeline monopoly Transneft has warned producers they may have to cut output following Ukraine's drone attacks on critical export ports and refineries, three industry sources said on Tuesday.Kyiv has stepped up attacks on Russian energy assets since August in a bid to impede Moscow's war effort in Ukraine and reduce the Kremlin's revenues as attempts to secure an end to the conflict through peace talks have stalled.Oil and gas revenues have accounted for between a third and half of Russia's total federal budget proceeds over the past decade…