UNITED NATIONS, Sept 20 (Reuters) - Russian President Vladimir Putin will only give up his "imperial ambitions" that risk destroying Ukraine and Russia if he recognizes he cannot win the war, German Chancellor Olaf Scholz said on Tuesday.

"This is why we will not accept any peace dictated by Russia and this is why Ukraine must be able to fend off Russia's attack," Scholz said in his first address to the United Nations General Assembly.

The return of imperialism, with Putin's war on Ukraine, was not just a disaster for Europe but for the global, rules-based peace order, the chancellor said. He called on the U.N. to defend this from those who would prefer a world where the "strong rule the weak".

"Do we watch helpless as some want to catapult us back into a world order where war is a common means of politics, independent nations must join their stronger neighbors or colonial masters, and prosperity and human rights are a privilege for the lucky few?" Scholz asked.

"Or do we manage together to ensure the multipolar world of the 21st century remains a multilateral world? My answer, as a German and European, is: we must manage it."

To better achieve this end, the global south needed a greater say in world affairs, Scholz said. With greater responsibility would come greater conviction, he said.

Western allies have been competing with Russia for diplomatic influence since the Kremlin began its invasion on Feb. 24.

Scholz announced Berlin would host a conference on the reconstruction of Ukraine on Oct. 25. Germany would help the Kyiv government with the "enormous cost of rebuilding the country", Scholz said.