NEW YORK: The sale of migrants into slavery in Libya may amount to crimes against humanity, the United Nations Security Council said on Thursday, voicing “grave concern” after footage appearing to show Africans being auctioned there sparked global outrage.

The 15-member council unanimously adopted a formal statement calling on Libyan authorities to investigate the reports of migrants being sold and bring the perpetrators to justice.

Libya’s UN-backed government last month said it would take action after a video broadcast by CNN, which appeared to show the auction of African migrants as farmhands for $400, led to protests across Europe and Africa.

“The Security Council expresses grave concern about reports of migrants being sold into slavery in Libya,” the statement said. “(It) condemns such actions as heinous abuses of human rights which may also amount to crimes against humanity.” Young African men bound for Europe are frequently caught in trafficking networks and sold for labour in Libya, where many migrants are detained, tortured, and even killed, according to the UN International Organization for Migration (IOM).

The IOM said last week that it was working with partners to try to empty the detention centres, condemned as inhumane by rights groups and estimated to hold as many as 20,000 migrants.

The Security Council also said the Libyan authorities should work with international organisations and UN agencies to ensure humanitarian access to detention centres in the country.

Hundreds of thousands of other migrants are believed to be in lawless Libya, and many of them are being held by smugglers under lock-and-key in a country consumed by factional violence since strong man Muammer Gaddafi was ousted six years ago.

“Reports that people escaping violence are being sold into slavery in Libya are horrifying,” US Ambassador to the United Nations, Nikki Haley, said in a statement. “All countries must do everything they can to end this barbaric practice.” The statement was adopted weeks after the Security Council unanimously backed a resolution urging tougher action to crack down on trafficking and modern slavery worldwide.

The resolution called on countries to adopt anti-trafficking laws, ramp up efforts to investigate and dismantle criminal networks, and provide greater support for survivors of slavery.

Libya’s prime minister vowed on Thursday to take action to ease the suffering of tens of thousands of migrants stranded in his north African country, even though it is still battling instability.

Thousands of migrants, many from sub-Saharan Africa, had travelled to Libya in the hopes of making the sea crossing to reach Italy, before moving northwards to other parts of Europe.

But they found themselves stuck in holding centres, as Tripoli worked with the European Union to cut off the sea smuggling routes.

Speaking after talks with German Chancellor Angela Merkel in Berlin, Libya’s Prime Minister Fayez al-Sarraj said the numbers of migrants are “staggering − we are talking about 500,000 migrants outside holding centres and 20,000 in the 42 centres run by the Interior Ministry.” But he said that last month’s two-day summit in Abidjan on the migration crisis has prompted some African countries to take action.

“Some African countries began to organise flights to repatriate their citizens and we are very open and cooperative with these countries,” Sarraj said, adding that Tripoli has also agreed with Rome to set up repatriation offices in migrant holding centres.

Agencies