MUNICH (Reuters) -Ukrainian President Volodymyr Zelenskiy urged allies at a global security conference on Saturday to plug an "artificial" shortage of weapons that is giving Russian forces the upper hand on the battlefield and said stalled U.S. aid was imperative. Addressing the gathering of politicians, diplomats and military officials from around the world, who gave him a standing ovation, Zelenskiy mixed gratitude for the support shown by Western countries with urging them for more. He spoke at a critical juncture in Russia's nearly two-year-old invasion of Ukraine, with his troops forced to withdraw from the devastated eastern town of Avdiivka.