Zelensky sought to rally support among world leaders at their annual gathering in New York weeks before a US election that could sharply shift the stance of Ukraine's main backer.

Speaking from the UN rostrum in a black polo jacket, Zelensky said that Ukrainian intelligence has found that Russia is scanning the country's nuclear infrastructure by satellite.

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," Zelensky said.

"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.

Russia in recent weeks 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 -

Zelensky on Thursday will head to the White House to see President Joe Biden and present what he describes as a "victory plan" that shows a path forward for Ukraine.

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: "You will not boost your power at Ukraine's expense... 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.

Chinese Foreign Minister Wang Yi told a Security Council session on Tuesday that diplomacy was the only solution.

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

Donald Trump, running again for president, called Zelensky the "greatest salesman on Earth."

"Every time Zelensky comes to the United States, he walks away with $100 billion," Trump said, claiming "we're stuck in that war unless I'm president."

The United States has provided around $175 billion in both military and economic assistance to Ukraine since Russia invaded in February 2022, and Biden has ruled out sending troops.

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.

Zelensky said he hoped to see Trump while in the United States and explain that the war is more complicated than Trump characterizes it as.

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 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.

The Security Council will hold a special session on Wednesday on Lebanon, as the United States says it hopes to present ideas for de-escalation.

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.