Zelensky Warns World Leaders That Russian Aggression Could Expand Beyond Ukraine

“Russia believes that the world will grow weary and allow it to brutalize Ukraine without consequence,” Mr. Biden said as President Volodymyr Zelensky of Ukraine watched from the audience. “But I ask you this: If we abandon the core principles” of the United Nations Charter “to appease an aggressor, can any member state in this body feel confident that they are protected? If we allow Ukraine to be carved up, is the independence of any nation secure? I respectfully suggest the answer is no.”

“We have to stand up to this naked aggression today to deter other would-be aggressors tomorrow,” Mr. Biden continued. “That’s why the United States, together with our allies and partners around the world, will continue to stand with the brave people of Ukraine as they defend their sovereignty and territorial integrity — and their freedom.”

Mr. Zelensky offered his own stirring speech not long afterward, arguing to the assembled leaders and diplomats that President Vladimir V. Putin’s war against Ukraine was a war against all of their nations as well. He accused Moscow of weaponizing food, energy and even children with dire effects not just in his country but in far-flung corners of the world.

“The goal of the present war against Ukraine is to turn our land, our people, our lives, our resources into a weapon against you, against the international rules-based order,” said Mr. Zelensky, speaking in English and wearing one of his trademark olive green military-style shirts. “We have to stop it,” he added. “We must act united to defeat the aggressor.”

He dismissed efforts to broker a peace deal without Ukraine’s involvement, what he called “shady dealings behind the scenes.” Characterizing Russia as an unreliable partner, he cited the recent death of Yevgeny Prigozhin, the mercenary leader who had defied Mr. Putin. “Evil cannot be trusted,” Mr. Zelensky said. “Ask Prigozhin if one bets on Putin’s promises.”

Both Mr. Biden and Mr. Zelensky received strong applause from some of the delegations in the hall, but many others did not clap. Mr. Putin, the target of an arrest warrant for war crimes issued by the International Criminal Court, did not come to New York for the annual opening session, but his envoy sat in Russia’s seat during Mr. Zelensky’s speech taking notes or looking down at his phone.

Mr. Zelensky was to address the U.N. Security Council on Wednesday with a plan to discourage war even after the fighting in his country eventually ends and then will head to Washington, where on Thursday he will meet with Mr. Biden at the White House, stop by the Pentagon and visit Capitol Hill to plead for continuing assistance.

Unlike his first wartime trip to Washington last winter, he will not address a joint meeting of Congress and will find more resistance among some far-right Republicans in the House who are trying to block Mr. Biden’s request for $24 billion more aid. Critics of further aid maintain that the war is not central to America’s national interest and the money would be better spent at home on border security or other priorities.

Speaker Kevin McCarthy responded sharply when asked about America’s commitment to Ukraine. “Is Zelensky elected to Congress? Is he our president? I don’t think I have to commit anything and I think I have questions for him,” he told reporters on Capitol Hill.

“Where’s the accountability on the money we’ve already spent?” he added. “What is the plan for victory? I think that’s what the American public wants to know.”

Mr. Biden has continued to provide aid using previously approved funds, and Defense Secretary Lloyd J. Austin III announced on Tuesday that American M1 Abrams tanks would be arriving in Ukraine soon. “That will add another formidable armor capability to join the weapons that are already on the battlefield,” he said in Germany after a meeting of about 50 countries supporting Ukraine. He added: “I salute Ukraine’s brave forces, and we’ve got their backs.” Ukraine’s fight, he continued, is “one of the great causes of our time.”

Mr. Biden’s speech came as other major leaders skipped the annual opening session of the General Assembly, including Mr. Putin and President Xi Jinping of China, effectively leaving the stage to the American president. He used the opportunity to reach out to the so-called global south — the traditionally unaligned developing nations that his advisers call the “swing states” of the foreign policy world — to enlist them to the American view of the threats that Russia and China pose to the international system.

While he took an unrelenting stance against Russia’s brutal war and warned against appeasing Moscow, he drew a more measured line on China, repeating his commitment to “push back on aggression and intimidation” by Beijing while seeking ways to work together and denying that he was trying to contain the Asian giant. “We seek to responsibly manage the competition between our countries so it does not tip into conflict,” he said.

Mr. Biden mentioned a litany of other major issues confronting the world today, like fentanyl abuse, artificial intelligence, terrorism, human rights, women’s rights, L.G.B.T. rights and arms control, without breaking much new ground on any of them. He stressed the dangers of climate change as he urged more action to combat it, citing heat waves, wildfires, drought and the flooding in Libya.

“Together, these snapshots tell an urgent story of what awaits us if we fail to reduce our dependence on fossil fuels and begin to climate-proof the world,” he said. Under his administration, he said, “the United States has treated this crisis as the existential threat from the moment we took office, not only for us, but for all of humanity.”

Mr. Biden will be using his time at the United Nations this week to meet with other world leaders. He met Tuesday afternoon with the leaders of the five Central Asian republics that used to be part of the Soviet Union — Uzbekistan, Kazakhstan, Kyrgyzstan, Tajikistan and Turkmenistan — the first time a president has sat down collectively with counterparts with those countries.

The “Stans,” as they are often called by diplomats, have been a key area of competition between Russia and China in the years since they gained their independence from the Soviet collapse, but the United States has sought influence there as well, particularly during its ill-fated war in Afghanistan. Mr. Biden’s meeting with their leaders is in keeping with his strategy of bolstering relations with nations in China’s neighborhood to counter assertive actions by Beijing.

On Tuesday evening, Mr. Biden and Jill Biden were to host a reception for other world leaders at the Metropolitan Museum of Art. On Wednesday, he was scheduled to sit down separately with Prime Minister Benjamin Netanyahu of Israel and President Luiz Inácio Lula da Silva of Brazil.

His meeting with Mr. Netanyahu will be their first in the United States since Mr. Biden became president, a much-delayed get-together that, tellingly, will not take place at the White House amid tension over Mr. Biden’s outreach to Iran and the Israeli leader’s efforts to diminish the power of courts in his country. In his speech on Tuesday, Mr. Biden touted his efforts to open diplomatic relations between Israel and Saudi Arabia while emphasizing his support for a two-state solution to Israel’s conflict with Palestinians.

While Russia and Ukraine were only a portion of his address, far less so than last year, they represented the core tension in the room as one of the United Nations’ permanent Security Council members was excoriated from the famed emerald green rostrum. Mr. Biden complained that Mr. Putin had been “shredding longstanding arms control agreements,” while Mr. Zelensky noted that his country had given up its nuclear weapons in exchange for a promise by Russia to respect its independence, a promise now violently broken.

Mr. Zelensky said that tens of thousands of Ukrainian children had been taken from occupied territory, sent to Russia and, he added, turned against their home country and relatives. “This is clearly a genocide,” Mr. Zelensky said. “When hatred is weaponized against one nation, it never stops there.”

“Each decade, Russia starts a new war,” he added, noting Moscow’s invasions and military interventions in Moldova, Georgia and Syria as well as its pressure on Belarus, Kazakhstan and the Baltic republics.

“War crimes must be punished, deported people must come home and the occupier must return to their own land,” Mr. Zelensky said before finishing with the historical national saying that has become a defiant mantra since the war began: “Slava Ukraini,” or “Glory to Ukraine.”