Ukrainian President Volodymyr Zelensky takes center stage at the United Nations on Wednesday as he seeks to sustain global support ahead of US elections that could bring a sharp shift from his top backer.

Zelensky, clad in his trademark military fatigues, a year ago made a dramatic first wartime appearance before the annual UN General Assembly as he urged international pressure on Russia over its invasion.

One year later, Zelensky may still enjoy star power, but Ukraine faces rising headwinds as Russian troops advance in the east and US Republican presidential candidate Donald Trump threatens to close the books on military aid.

Zelensky will address the General Assembly on another hectic day of international diplomacy that will include an emergency Security Council meeting on the Lebanon crisis.

Speaking to a Security Council session on Tuesday on Ukraine, Zelensky brushed aside calls to negotiate a settlement with Russian President Vladimir Putin.

"Russia can only be forced into peace, and that is exactly what's needed," Zelensky said.

Zelensky will head Thursday to Washington to enjoy a show of support at the White House by President Joe Biden and to present a further outline of what the Ukrainian leader calls a "victory plan."

'Greatest salesman'?

Former US president Trump on Wednesday called Zelensky the "greatest salesman on Earth."

"Every time Zelensky comes to the United States, he walks away with $100 billion," Trump said.

"But we're stuck in that war unless I'm president. I'll get it done. I'll get it negotiated, I'll get out. We got to get out."

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 for the first time over delaying aid to Ukraine to press Zelensky to dig up dirt on Biden.

Zelensky said he would likely meet Trump while in the United States and explain that the war is more complicated.

"My feeling is that Trump doesn't really know how to stop the war even if he might think he knows how. With this war, oftentimes, the deeper you look at it the less you understand," Zelensky told The New Yorker magazine.

Zelensky was harsher in his assessment of Trump's running mate, J.D. Vance, who has said bluntly that he does not care about Ukraine and that the United States should focus instead on confronting China.

"Let Mr. Vance read up on the history of the Second World War, when a country was forced to give part of its territory to one particular person," Zelensky told The New Yorker. 

Referring to the Holocaust, Zelensky voiced hope that Jewish groups would remind Vance how "millions perished thanks to the fact that someone offered to give up a sliver of territory."

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

But support by most Western nations remains strong for the moment. The European Union last week announced 35 billion euros ($39 billion) in loans backed by the revenues of frozen Russian assets.

Britain has been among the most robust supporters of Ukraine. Foreign Secretary David Lammy told AFP that his government was committed to helping "to put Ukraine in the strongest possible position" as winter approaches.

Lebanon crisis

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

French President Emmanuel Macron on Tuesday met Iran's new president, Masoud Pezeshkian, and urged him to use his influence to rein in Hezbollah, which has been firing rockets for months at Israel in solidarity with Palestinian militant group Hamas.

Wednesday will also see talks at the UN on two other hot spots -- Sudan and Haiti.

The United States is looking for ways to drum up long-term financial support for stabilization efforts in violence-ravaged Haiti after Kenya began a long-awaited policing mission.