President Barack Obama criticized Russia over its alleged incursions into Ukraine during a speech at the United Nations Wednesday, taking his hardest stance yet after months of conflict in Eastern Europe and sanctions against Russian business leaders. Obama said Russia has challenged the "post-war order" that the U.N. was founded on and urged world leaders to stand on the right side of history.

"Russian aggression in Europe recalls the days when large nations trampled small ones in pursuit of territorial ambition,” Obama said. "We call upon others to join us on the right side of history – for while small gains can be won at the barrel of a gun, they will ultimately be turned back if enough voices support the freedom of nations and peoples to make their own decisions."

Obama said the U.S. will continue to back Ukraine and NATO allies, particularly in Eastern Europe and the Baltics, where leaders have expressed concern over alleged Russian action in Ukraine. European leaders approved the NATO Readiness Action Plan intended to halt Russian aggression against NATO members during a summit earlier this month.

Obama urged Russian leaders to work across borders for a diplomatic solution in Ukraine and reminded the U.N. of the achievements of cooperation between Russian and U.S. officials, particularly in the years following the collapse of the Soviet Union.

"A different path is available – the path of diplomacy and peace and the ideals this institution is designed to uphold," Obama said. "The recent cease-fire agreement in Ukraine offers an opening to achieve that objective. If Russia takes that path – a path that for stretches of the post-Cold War period resulted in prosperity for the Russian people – then we will lift our sanctions and welcome Russia’s role in addressing common challenges."

Russian President Vladimir Putin is not scheduled to make an appearance at the 69th session of the U.N. Russian Foreign Minister Sergey Lavrov and Russian Ambassador to the U.N. Vitaly Churkin represented the Russian delegation during Obama's remarks. Russia’s leadership was scheduled to address the U.N. Saturday around noon. Ukraine’s delegation will speak Thursday morning.