US President Joe Biden called on the world to rally against Russia's invasion of Ukraine in a speech to the UN General Assembly on Wednesday. Moscow's aggression against its neighbour is a violation of the UN Charter established in 1945 after World War II.

World leaders met at the United Nations in New York to condemn Russia's invasion of Ukraine, as Moscow-based leaders of four Ukrainian-occupied territories announced a referendum on joining Russia in the coming days.

U.N. members, aiming to "save future generations from the menace of war" and to provide conditions for nations to "live together in peace," pledge to refrain in their international relations from "threats or use of force against the territorial integrity or political independence of any State, or in any other way inconsistent with the purposes of the United Nations." the charter says.

In the apparently coordinated move, pro-Russian figures announced referendums for Sept. 23-27 in Luhansk, Donetsk, Kherson and Zaporizhzhia provinces, representing around 15% of Ukrainian territory, or an area about the size of Hungary.

"Ukraine has every right to liberate its territories and will continue to liberate them no matter what Russia has to say," Ukrainian Foreign Minister Dmytro Kuleba tweeted.

"The Russians can do whatever they want. Nothing will change that," he said in response to questions from reporters at the United Nations on Tuesday.

German Chancellor Olaf Scholz said Putin will only give up his "imperial ambitions" that risk destroying Ukraine and Russia if he recognizes he cannot win the war.

"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 General Assembly.

Japanese Prime Minister Fumio Kishida told the assembly that the UN's credibility was at risk due to Russia's invasion of a permanent member of the Security Council and that reforms of the UNSC were needed.

Russia's occupation of Ukraine is a behaviour that tramples the philosophy and principles of the UN Charter. This should never be tolerated, Kishida said.

White House national security adviser Jake Sullivan said Biden would not use his speech to argue that Russia should be ousted as a member of the U.N. Security Council, where it is one of five permanent members, capable of vetoing any security resolution.

But he said Biden will urge all nations to see that Russia's war against Ukraine is a violation of the U.N.'s 1945 charter signed by the countries that fought the Axis Powers in World War Two.

Russian President Vladimir Putin says the West wants to destroy Russia, that economic sanctions imposed on Moscow in response to the conflict are akin to a declaration of economic war and that Russia will build ties with other powers such as China and India. The Ukraine war was necessary because the expansion of the U.S.-led Western military alliance NATO represented an existential threat to Russia, Putin has said.

"The situation on the front clearly indicates the initiative is with Ukraine," Ukrainian President Volodymyr Zelenskiy said in a video address released early on Wednesday.

Ukraine's position did not change because of "some noise" from Russia, Zelenskiy added in a reference to the referendums.

In Kherson, where the regional capital is the only major city Russia has so far captured intact since the invasion, Ukraine has launched a major counter-offensive.

In the south, Russia controls most of Zaporizhzhia but not its regional capital.

Vladimir Rogov, a Russian-appointed official in Zaporizhzhia, said becoming part of Russia would help solve the conflict more quickly.

"This will show people with full clarity that Russia is here to stay, forever," he said on Telegram. "And they will take the necessary decision more quickly, lay down their weapons and cross over to the side of their people, their countrymen."

But Indian Prime Minister Narendra Modi told Putin last week that now is not the time for war, and Chinese President Xi Jinping expressed questions and concerns about the situation last week when the two leaders met.