Volodymyr Zelensky vowed in a video address on Wednesday that his forces would not stop until they had reclaimed all of Ukraine.
“We can return the Ukrainian flag to our entire territory. We can do it with the force of arms,” he said. “But we need time.”
The speech was delivered just hours after Russia’s president Vladimir Putin said it would mobilise as many as 300,000 reservists for the war effort.
It was the first time Mr Zelensky addressed the world’s leaders gathered together since Russia invaded in February.
Shortly after Putin’s announcement, flights out of the country rapidly filled up, and more than 1,000 people were arrested at anti-war demonstrations across the country.
Mr Zelensky didn’t discuss the developments in detail, but he suggested any Russian talk of negotiations was only a delaying tactic.
“They talk about the talks but announce military mobilisation.
“They talk about the talks but announce pseudo-referendums in the occupied territories of Ukraine,” he said.
Mr Zelensky asserted that Moscow wants to spend the winter preparing its forces in Ukraine for a new offensive, or at least preparing fortifications while mobilising more troops in the largest military conflict in Europe since World War II.
“Russia wants war. It’s true. But Russia will not be able to stop the course of history,” he said, declaring that “mankind and the international law are stronger” than what he called a “terrorist state”.
“He is desperately trying to claim the mantle of democracy for a regime without human rights or freedoms and he is making yet more bogus claims and sabre-rattling threats.
“This will not work. The international alliance is strong and Ukraine is strong.”
US President Joe Biden’s address, too, focused heavily on the war in Ukraine.
“This war is about extinguishing Ukraine’s right to exist as a state, plain and simple, and Ukraine’s right to exist as a people.
“Whoever you are, wherever you live, whatever you believe, that should make your blood run cold,” he said.
“If nations can pursue their imperial ambitions without consequences, then we put at risk everything this very institution stands for. Everything.”
The war will remain the main focus at the gathering on Thursday, when the Security Council plans to take up the issue.
Putin, who is not attending the General Assembly, has said he sent his armed forces into Ukraine because of risks to his country’s security from what he considers a hostile government in Kyiv; to liberate Russians living in Ukraine — especially its eastern region of the Donbas — from what he views as the Ukrainian government’s oppression; and to restore what he considers to be Russia’s historical territorial claims on the country.