Mauritanian authorities refused to issue entry visas to the dozen activists who had arrived in the capital, Sneiba El Kory, an official with SOS Esclaves, a local anti-slavery NGO, told AFP.
Officially, slavery was outlawed in Mauritania in 1981 but the west African country remains a bastion of the practice.
The American activists left the country Friday night on a European airline after waiting for several hours at the airport, a Mauritanian security source told AFP, without giving further details.
“The United States is disappointed and concerned with the decision to deny entry to this delegation,” the US embassy said in a statement released late Friday.
Mauritanian authorities did not respond to requests for comment.
The refusal to issue visas confirms that the Mauritanian government has something to hide,” said SOS Esclaves vice president Ahmed Ould Weddia.
The US activists were set to be in Mauritania for a week on a trip organised by a Chicago-based anti-slavery group which is part of US pastor Jesse Jackson’s Rainbow/PUSH Coalition.
Slavery is deeply entrenched in the vast, largely desert nation where light-skinned Berber Arab Moors enslaved local black populations after settling in Mauritania centuries ago.
Modern-day slavery under a hereditary system of servitude forces members of the “slave” caste to work without pay as cattle herders and domestic servants, despite an official ban.
Many Mauritanians live below the poverty line while there is huge disparity between the Arabised Moorish elites and the country’s black population.
A new law adopted in 2015 doubling prison terms for offenders has been hailed as a sign the government is finally getting serious on cracking down on the practice which activists say is widespread.
The Australia-based Walk Free Movement estimated in its 2014 Global Slavery Index that there were 156,000 slaves in Mauritania, or some four percent of the population.
