Zelensky addressed the annual gathering of world leaders to rally support before a high-profile visit Thursday to the White House, where President Joe Biden's administration promised new military aid.

Speaking from the UN rostrum in a black polo jacket, Zelensky said that Russian President Vladimir Putin "does seem to be planning attacks on our nuclear power plants and the infrastructure, aiming to disconnect the plants from the power grid."

"Any critical incident in the energy system could lead to a nuclear disaster. A day like that must never come," Zelensky said.

"Moscow needs to understand this, and this depends in part on your determination to put pressure on the aggressor," he told the General Assembly.

Shortly after his remarks, Putin made his most explicit threat yet to use nuclear weapons, saying Russia would consider such a response to a "massive" air attack on its soil.

Ukraine has been pushing the United States and its allies to ease restrictions on weapons that can strike deeper into Russia.

Russia captured the giant Zaporizhzhia nuclear plant soon after its invasion of Ukraine in February 2022.

In recent weeks it has been pounding Ukraine's electricity grid, in what Western and Ukrainian officials describe as an attempt to leave the country shivering during the winter.

- 'Never accept' deal from outside -

In his UN address, Zelensky singled out China and Brazil as he questioned the "true interest" of countries that have been pressing Ukraine to negotiate with Russia.

Employing the language of the Global South, Zelensky said: "The world has already been through colonial wars and conspiracies of great powers at the expense of those who are small."

"Ukrainians will never accept -- will never accept -- why anyone in the world believes that such a brutal colonial past, which suits no one today, can be imposed on Ukraine now," Zelensky said.

In response, Brazilian President Luiz Inacio Lula da Silva said Wednesday that a peace deal is the only way for Ukraine to "survive" the war.

"Only peace will guarantee that Ukraine survives as a sovereign country and Russia survives," he said at a press conference in New York.

Zelensky last year flew to the General Assembly in a dramatic first wartime appearance. But while he still maintains star power, the political landscape has changed.

Donald Trump, running again for president in a close race against Kamala Harris, called Zelensky "probably the greatest salesman on Earth."

"We continue to give billions of dollars to a man who refuses to make a deal, Zelensky," the Republican candidate told a campaign rally in North Carolina.

Republicans were livid after Zelensky told The New Yorker magazine that Trump and his running mate J.D. Vance did not understand the war's complexity.

The United States has provided around $175 billion in both military and economic assistance to Ukraine during the war. The Biden administration has ruled out sending troops.

The Biden administration announced another $375 million on Wednesday including munitions for HIMARS precision rocket launchers, cluster munitions and light tactical vehicles.

Trump in the past has voiced admiration for Putin and, during his 2017-2021 presidency, was impeached over delaying aid to Ukraine to press Zelensky to dig up dirt on Biden.

In Germany, the second-largest contributor of military aid to Ukraine, Chancellor Olaf Scholz is also facing pressure from parties opposed to support for Kyiv.

Prime Minister Keir Starmer, of Kyiv ally Britain, told the Security Council on Wednesday that Putin has been sending its citizens into a meat grinder and asked how Russia "can show its face" at the UN headquarters.

- Lebanon crisis -

The annual UN General Assembly extravaganza marks a swansong for Biden, 81, who has passed the torch to Vice President Kamala Harris to face Trump in the November 5 election.

The summit comes against a backdrop of chaos in the Middle East as Israel ramps up attacks on the Iran-backed Lebanese militia Hezbollah, killing hundreds and prompting a mass exodus of people.

Biden and French President Emmanuel Macron met to discuss a push for a ceasefire in Lebanon.

Wednesday also saw talks at the UN on two other hotspots -- Sudan and Haiti.

The United States announced millions in new assistance both for war-ravaged Sudan's humanitarian crisis and for stabilization efforts in violence-wracked Haiti.