In a report published on Thursday, three U.N.-appointed experts said they had found practices in U.S. prisons that amounted to "an affront to human dignity" in visits in April and May.

The U.S. diplomatic mission in Geneva declined to comment. The Federal Bureau of Prisons said it was committed to ensuring the safety and security of incarcerated individuals as well as employees and the public.

One such practice is restraining and shackling women prisoners during childbirth, the report said.

The experts "heard, first hand, unbearable direct testimonies of pregnant women shackled during labour, who due to the chaining, lost their babies", it said. Asked to give details, a U.N. rights spokesperson referred to "several" cases and confirmed they all involved Black women.