Donald Trump has criticised other countries' migration and climate policies, during a scathing speech to the United Nations that also took aim at the body itself.

The US president told the UN General Assembly in New York it was time to end the "failed experiment of open borders" and claimed UN predictions over climate change were wrong, prompting gasps from the assembly floor.

Trump also made a major shift in his position on the war with Russia, saying Kyiv can "win all of Ukraine back in its original form".

In a post on his Truth Social platform, he said Ukraine could get back "the original borders from where this war started" with the support of Europe and Nato.

In the wide-ranging UN speech stretching to almost an hour, Trump repeated claims that he had stopped "seven wars" since returning to the White House, and accused the UN of failing to help him do so.

He queried the UN's purpose and said it was "not living up to its potential".

Though the reaction from delegates was largely muted, some were visibly squirming in their seats as the US president addressed the UN for the first time since he began his second term.

Several could be seen shaking their heads as he denied climate change, calling it the "greatest con job ever perpetrated on the world", and refuted the use of renewable energy.

"The entire globalist concept, asking successful industrialised nations to inflict main on themselves and radically disrupt their entire societies must be rejected completely and totally," he told his audience.

The overwhelming majority of scientists and experts agree that climate change is caused by humans, and is real.

Trump also levelled criticism at UN assistance for asylum seekers, saying the organisation was "funding an assault on western countries" by offering cash support.

He singled out Europe, saying it was in "serious trouble" over migration.

"We have a big heart for places that are struggling – we have to solve the problem in their countries," he said.

Trump falsely claimed that London wanted to move to follow Sharia law - an Islamic legal system - and singled out the city's mayor Sadiq Khan for criticism.

A spokesperson for Khan told the BBC: "We are not going to dignify his appalling and bigoted comments with a response."

Trump's speech also referenced the wars in Gaza and Ukraine. He said a ceasefire was needed in Gaza but criticised the move from some countries to recognise a Palestinian state, saying it "would be a reward" for Hamas.

On Ukraine, Trump said on Truth Social that his position had changed "after getting to know and fully understand the Ukraine/Russia military and economic situation".

He posted the message after talks with Ukrainian President Volodymyr Zelensky, held after Trump had addressed the UN General Assembly.

His latest backing for Kyiv to take back the entirety of Russian-occupied territories follows previous warning that any peace process would likely involve Ukraine giving up some territory, an outcome Zelensky has consistently rejected.

Meanwhile Trump said it was "embarrassing" that some European nations were buying oil and gas from Russia even as they oppose its full-scale invasion of Ukraine. The war was also "making Russia look bad", he said.

After the address, Trump said he believed the speech had been "very well received" in a post on his Truth Social platform.

He had a meeting with UN Secretary General Antonio Guterres after the speech - the first of his second term - during which he said the US was "behind the United Nations 100%" even though he "may disagree with it sometimes".

Guterres told Trump he believed there were "many areas where we can cooperate in a positive way, and I would think that the most important is peace".

Trump was also set to meet officials from several Middle Eastern countries, including Qatar, Saudi Arabia, and Egypt, before hosting a reception for more than 100 world leaders on Tuesday.