Nikki Haley, the US ambassador to the United Nations, has launched a scathing attack on the UN monitor on extreme poverty, dismissing his recent report on America that accuses Donald Trump of cruelly forcing millions of citizens into deprivation as “misleading and politically motivated”.

Haley, the former Republican governor of South Carolina, said she was “deeply disappointed” that the UN special rapporteur on extreme poverty and human rights, Philip Alston, had “categorically misstated the progress the United States has made in addressing poverty … in [his] biased reporting”. She added that in her view that “it is patently ridiculous for the United Nations to examine poverty in America” – which prompted puzzlement as Alston carried out his investigation at the formal invitation of the Trump administration.

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Instead, Haley suggested, the UN monitor should have used his voice “to shine a light” on countries where governments were causing pain and suffering on their own people, such as Burundi and the Democratic Republic of Congo. “The special rapporteur wasted the UN’s time and resources, deflecting attention from the world’s worst human rights abusers and focusing instead on the wealthiest and freest country in the world.”

Haley’s remarks deepen the dispute between the Trump administration and the UN human rights council in Geneva that has been blazing all week. Alston will be presenting his report on US poverty to the council on Friday, laying out his key findings to its 46 members and to an empty chair where the US has traditionally sat.

Play Video 0:53 Nikki Haley: US will lead on human rights outside 'misnamed' UN council – video

On Tuesday, Haley announced that the US was pulling out of membership of the human rights council, describing it as a “cesspool of political bias”. It marked the first time that any state has withdrawn from the council since its inception in 2006.

Haley made her counterpunch to the UN monitor – the first substantive comments to emerge from the Trump administration over Alston’s devastating critique of its pro-rich and anti-poor policies – in a response to Bernie Sanders. The Vermont senator had led a joint plea on the back of the UN report from 20 prominent members of Congress, including the senators Elizabeth Warren and Cory Booker and Georgia representative John Lewis, calling on Trump to work with them to tackle “massive levels of deprivation and the immense suffering this deprivation causes”.

Sanders on Thursday issued a further response to Haley’s attack on the UN rapporteur. The 2016 runner-up for the Democratic presidential nomination agreed with Haley that Burundi and the Democratic Republic Congo faced far worse problems, but pointedly remarked that America’s poverty was taking place “in the richest country in the history of the world and a time when wealth and income inequality is worse than at any time since the 1920s”.

Sanders said it was appropriate for the UN to focus on America, given that 40 million people in the US still live in poverty, more than 30 million have no health insurance, and 40% of Americans cannot afford $400 in an emergency.

“I hope you will agree that in a nation in which the top three people own more wealth than the bottom half, we can and must do much better than that,” Sanders said.

The sharpness of tone in Haley’s criticisms of the UN rapporteur raises questions about whether the timing of the Trump administration’s decision to quit the UN human rights council was motivated in part by irritation over Alston’s decision to put his spotlight on American inequality. Perceived anti-Israel bias within the council, and the ongoing inclusion among its members of states with poor human rights records such as the Democratic Republic of Congo and Venezuela, were clearly the main driving factors behind the pullout, but it is also clear that Haley was also peeved by the directness of Alston’s conclusions.

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The rapporteur carried out a 10-day tour in December of poverty hotspots in the US, from California, though Alabama and West Virginia, to Puerto Rico. He concluded that though levels of hardship had been high for decades within America, Trump was taking it to another level by steering the country towards a “dramatic change of direction” that was rewarding wealthy Americans while stripping vulnerable Americans of welfare protections.

In a press conference on Thursday, on the eve of presenting his report on the US to the council, Alston said he had “no idea” whether his investigation had reinforced or influenced the timing of the US government’s withdrawal. But he added that the move was “highly regrettable. I think it’s significant that of the 47 members of the council, only one has chosen to leave.”

He added: “There is a fear that this is part of a broader attack on human rights and multilateralism.”