The US is acting with impunity and believes its power matters more than international law, the head of the UN has told the BBC.

Speaking to BBC Radio 4's Today programme, António Guterres said Washington's "clear conviction" was that multilateral solutions were irrelevant.

What mattered, he continued, was the "exercise of the power and influence of the United States and sometimes in this respect by the norms of international law".

His comments come weeks after the US struck Venezuela and seized its president, and in the context of Donald Trump's repeated threats to annexe Greenland.

Guterres said he believed the UN's founding principles - including the equality of member states - were now under threat.

President Trump has previously been scathing in his criticism of the United Nations.

He used his address at last September's General Assembly to question its very purpose, claiming he had "ended seven unendable wars" on his own and the UN "did not even try to help in any of them".

"Later I realised that the UN wasn't there for us," he said.

Presented with this damning assessment, Guterres admitted his organisation was struggling to make members abide by the international laws laid out in the UN Charter.

The UN was "extremely engaged" in solving major global conflicts, he insisted. "But the UN has no leverage - the big powers have stronger leverage.

He questioned whether that extra leverage was being used to produce real and enduring solutions to those conflicts, or just quick fixes. "There is a big difference between the two things," he noted.

Guterres also said his organisation needed reform to tackle "dramatic problems and challenges" facing its 193 members.

"There are those who believe the power of law should be replaced by the law of power," the UN chief said.

"Indeed, when one sees the present policy of the United States, there is a clear conviction that multilateral solutions are not relevant and that what matters is the exercise of the power and the influence of the United States and sometimes, in this respect, by the norms of international law".

He suggested the UN Security Council - designed to maintain international peace and security - no longer represented the world and was "ineffective".

Any one of the council's permanent members - France, China, Russia, the UK or US - can currently veto resolutions. Both Russia and the US have used this power to frustrate global efforts to end the wars in Ukraine and Gaza.

Guterres claimed vetoes were being used to further individual members' interests, and criticised the fact that "three European countries" were permanent members.

He called for changes to the council's composition - to "regain legitimacy" and "give voice to the whole world" - and to limit veto powers to avoid unacceptable "blockages".

Guterres – a former Portuguese prime minister – took on the role of UN Secretary-General in 2017 and will leave the role at the end of this year.

In his annual remarks to the General Assembly, traditionally used to identify the organisation's priorities, he warned of a world in chaos, "brimming with conflict, impunity, inequality and unpredictability", and identified "brazen violations of international law" as one of the biggest challenges facing the global order.

One of the ongoing conflicts Guterres identified as key for the UN was Gaza.

For large parts of the war, it was prevented from distributing aid in the strip, as Israel blocked international humanitarian organisations from bringing it into the territory.

At one point, Israel even backed an external contractor – the Gaza Humanitarian Organisation – to do the work the UN had traditionally carried out for decades. Hundreds of Palestinians were killed trying to access food at GHF sites.

When asked if he saw the UN as powerless in Gaza, where it should have made a difference, he said: "Of course, but let's be clear.

"For a long time, Israel was saying that humanitarian aid was not distributed because the UN was not able to do so. Of course, whenever Israel would not allow us to move into Gaza, we couldn't move into Gaza. And then there was a ceasefire, and a massive flow of humanitarian aid."

He stressed: "We were ready, provided we had the conditions."

A few days ago, Guterres told the UN General Assembly that "1945 problem-solving" wouldn't solve 2026 problems, referring to the organisation's founding structure.

The challenges keep coming, with the US-forced change of leadership in Venezuela, regime forces in Iran killing thousands of protestors and Donald Trump's stated aim of annexing Greenland.

Questions about the death of multilateralism - and the failure of some world leaders to speak out and defend the rule of international law - are growing ever-louder.

But Antonio Guterres told me he was remaining positive.

"I think that people are sometimes reluctant to confront the powerful. But the truth is that if we don't confront the powerful, we will never be able to have a better world."