UN human rights experts say US President Donald Trump's travel ban on nationals from seven Muslim-majority states contravenes international law and could lead to people denied asylum being sent home to face torture.
Mr Trump's executive order curbing immigration has caused an international outcry, even among US allies, along with chaos and bewilderment among travellers. Legal challenges have spread with three US states suing to overturn the order, saying it flouts constitutional guarantees of religious freedom.
In a statement, the UN experts urged the Trump administration to protect people fleeing war and persecution and uphold the principle of non-discrimination based on race, nationality and religion.
The United States should not force back refugees, a practice known as refoulement, they said.
"Such an order is clearly discriminatory based on one's nationality and leads to increased stigmatisation of Muslim communities," said the experts.
"Recent US policy on immigration also risks people being returned, without proper individual assessments and asylum procedures, to places in which they risk being subjected to torture and other cruel, inhuman or degrading treatment, in direct contravention of international humanitarian and human rights laws which uphold the principle of non-refoulement."
The independent experts included the UN special rapporteurs on migrants, Francois Crepeau; on racism, Mutuma Ruteere; on human rights and counter-terrorism, Ben Emmerson; on torture, Nils Melzer; and on freedom of religion, Ahmed Shaheed.
UN High Commissioner for Human Rights Zeid Ra'ad al-Hussein said on Monday that discriminating against people on the basis of their nationality is illegal.
The UN experts voiced concern that people travelling to the United States could be subject to detention for indefinite periods and ultimately deported. They called on Washington to live up to internationally agreed obligations to offer refuge to those fleeing persecution and conflicts.
Mr Melzer also urged Mr Trump not to consider returning to waterboarding and other methods of torture as interrogation techniques used during George W. Bush's administration but banned by his Democratic successor Barack Obama. Mr Trump has said he believes waterboarding works but his top defence and security appointees have said they would oppose any use of it.