Large numbers of Nigerians have been trapped in Libya where they were trying to cross to Italy by sea, but were stopped by local armed factions and the Libyan coastguard.
Nigerian officials on a fact-finding mission to Libya expressed shock at what they saw and heard from victims.
"They talked about various abuse - systematic, endemic, and exploitation of all kinds," said Nigeria's Foreign Affairs Minister Geoffrey Onyeama. "There were obviously interests that wanted to keep as many of them there as possible because they were commodities."
Al Jazeera's Ahmed Idris, reporting from the Libyan capital Tripoli, said Nigerians there told of abuses such as slavery, rape, imprisonment, and torture.
"These happened either in the hands of the authorities or people-smugglers," Idris said.
"The journey back home for them is a mixed bag. A lot of them are happy that they are free at last, but disappointed that many lost so much in this country and they are going back with nothing."
Thousands to be evacuated
Citizens of Nigeria, the most-populated country on the African continent, have been the largest group of migrants travelling to Libya to try and cross the Mediterranean Sea to Europe.
Nigeria's government said the rescue flights will continue for as long as necessary, estimating that about 5,500 people would be flown back to their country.
"If I'll die, I want to die in my country. I have suffered so much in the last few months after I left my great country of Nigeria," a migrant waiting to fly back home told Al Jazeera, warning others not to make the same mistake he made.
Another said he wanted to go to Italy when he reached Libya. "But now in this situation, I want to go back to my country."
The UN's International Organization for Migration said 171,635 migrants and refugees entered Europe by sea during 2017, with nearly 70 percent arriving in Italy. The remainder were divided among Greece, Cyprus and Spain.
This compared with 363,504 arrivals during the same period in 2016, according to the agency.