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 annex Greenland.

Guterres said he believed the founding principles of the UN - 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.

Guterres admitted his organization was struggling to make members abide by 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.

Guterres also mentioned needed reforms to tackle the dramatic problems and challenges facing the UN's 193 members.

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

Guterres claimed vetoes were used to further individual members' interests, criticizing 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.

Guterres stressed the importance of reforming the council to avoid unacceptable blockages in global governance.

As his term as secretary general nears its end, Guterres has expressed hope for a better system that can effectively address today’s global challenges.