The head of the U.N. migration agency says the United Nations plans to try to empty government-controlled detention centers in Libya that hold about 15,000 people as part of a stepped up effort against the smuggling and slavery of migrants.

William Lacy Swing spoke Tuesday at an emergency meeting of the U.N. Security Council called by France following video footage last week of African men being sold at a slave market in Libya. He said "all of us feel the pressure that this is an enormous human tragedy and we can stop it."

Swing said emptying the 30 or so detention centers will require agreement from Libya and African and European countries.

If everyone is willing to help, he said, the centers can be emptied "in a very short order."