UNITED NATIONS -- U.N. Secretary-General Antonio Guterres is calling for an immediate investigation into the reported sale of African migrants in Libya, saying transactions may amount to crimes against humanity.

Guterres said Monday that "slavery has no place in our world and these actions are among the most egregious abuses of human rights."

"I abhor these appalling acts and call upon all competent authorities to investigate these activities without delay and to bring the perpetrators to justice. I have asked the relevant United Nations actors to actively pursue this matter," Guterres said in a statement.

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The former U.N. refugee chief said he was "horrified" at video footage, broadcast on CNN, of the bidding and sale of migrant men seeking a better life.

He said migration must be addressed "in a comprehensive and humane manner" and should include a crackdown on smugglers and traffickers.