BRUSSELS (Reuters) - European Energy Commissioner Guenther Oettinger warned on Tuesday he was not ruling out "worst case scenarios" on Europe's energy security due to Russian President Vladimir Putin's "lies" amid Moscow's actions in Ukraine.

"That Putin would use false information, lies and weapons was beyond my imagination," Oettinger said at an event of German energy utility RWE in Brussels.

"That's why I am not ruling out worst case scenarios any more," he added, referring to Europe's energy security.

The war in Ukraine, which has led to the worst stand-off between Russia and the West since the Cold War, has heightened fears in Europe of gas shortages in the winter.

Russia is Europe's biggest supplier of oil, coal and natural gas, and its pipelines through Ukraine are currently subject to political manoeuvring, not for the first time, as the West and Moscow clash over the latter's military action in Ukraine.

Oettinger is mediating talks between Russia and Ukraine to resolve a gas pricing row and avert a damaging supply cut in the winter.

On Tuesday he said that he feared the advance of pro-Russian separatists toward the port city of Mariupol in the south of Ukraine could create a corridor stretching from there to Crimea, which Russia annexed in March, and Russian-speaking breakaway state Transnistria.

"Then Ukraine would be cut off from the Black Sea."

EU countries and the United States have responded to Russia's actions in Crimea and Ukraine by imposing economic sanctions on Russia and are set to tighten them further this week.


(Reporting by Julia Fioretti and Tom Koerkemeier; Editing by Lisa Shumaker)