Libya’s U.N.-backed government has said it is investigating.
“The situation in Libya, of people being sold into slavery, is appalling and unacceptable. We will do everything to protect our citizens wherever they might be,” Buhari wrote on his Twitter account.
Nigeria had “started bringing back home all Nigerians stranded in Libya and elsewhere,” he added.
The U.N.’s International Organization for Migration said 239 Nigerians flew home from Tripoli on Tuesday.
It has said Nigerian migrants risk exploitation, detention and abuse as they head north to Libya, hoping to cross the Mediterranean to Europe.