Poverty in the United States is extensive and deepening under the Trump administration whose policies seem aimed at removing the safety net from millions of poor people, while rewarding the rich, a UN human rights investigator has found.

Philip Alston, UN special rapporteur on extreme poverty, called on US authorities to provide solid social protection and address underlying problems, rather than "punishing and imprisoning the poor".

The Australian, a veteran UN rights expert and New York University law professor, will present his report to the United Nations Human Rights Council later this month.

Welfare benefits and access to health insurance are being slashed, but President Donald Trump's tax reform has awarded "financial windfalls" to the mega-rich and large companies, further increasing inequality, he said in the report.

US policies since President Lyndon Johnson's war on poverty in the 1960s have been "neglectful at best", he said.

"But the policies pursued over the past year seem deliberately designed to remove basic protections from the poorest, punish those who are not in employment and make even basic health care into a privilege to be earned rather than a right of citizenship," Alston said.

Almost 41 million people or 12.7 per cent live in poverty, 18.5 million in extreme poverty, and children account for one in three poor, he said. The US has the highest youth poverty rate among industrialised countries, he added.

"Its citizens live shorter and sicker lives compared to those living in all other rich democracies, eradicable tropical diseases are increasingly prevalent and it has the world's highest incarceration rate ... and the highest obesity levels in the developed world," Alston said.

The data is based on his mission in December to several US states, including rural Alabama, a slum in downtown Los Angeles, California, and the US territory of Puerto Rico.

A US official in Geneva, asked for comment, told Reuters: "The Trump Administration has made it a priority to provide economic opportunity for all Americans."

Citing "shameful statistics" linked to entrenched racial discrimination, Alston said that African Americans are 2.5 times more likely than whites to live in poverty and their unemployment rate is more than double. Women, Hispanics, immigrants, and indigenous people also suffer high rates.

At least 550,000 people are homeless in America, he said.

The tax overhaul has permanently cut the top corporate rate to 21 per cent from 35 per cent. Tax cuts for individuals, however, are temporary and expire after 2025.

Alston dismissed allegations of widespread fraud in the welfare system and criticised the US criminal justice system. It sets large bail bonds for a defendant seeking to go free pending trial, meaning wealthy suspects can afford bail while the poor remain in custody, often losing their jobs, he said.

"There is no magic recipe for eliminating extreme poverty and each level of government must make its own good-faith decisions. At the end of the day, however, particularly in a rich country like the United States, the persistence of extreme poverty is a political choice made by those in power," he said.