Humans are being bought and sold at modern-day slave auctions in Libya, according to an investigation by CNN.

The news channel found evidence of two separate auctions, held after dark at undisclosed locations in the war-ravaged country, which has become the epicentre of the ongoing migrant crisis.

Footage showed men, the majority from Niger, being auctioned off by local slavers for the equivalent of $400 each (£305).

Reporters at CNN were given mobile phone footage showing one group of men being sold at an indoor auction, which they said they were able to independently verify.

CNN slave investigation phone footage.

Foto: Mobile phone footage provided to CNN shows two men being sold in an auction, while a salesman points at them and talks prices. source CNN

Later they travelled to Tripoli, Libya, and were directed to an auction which they managed to film in person.

Reporter Nima Elbagir said she saw 12 men sold off in a matter of minutes by a man dressed in camouflage gear, who referred to them as "merchandise."

He reportedly said: "Does anybody need a digger? This is a digger, a big strong man, he'll dig. What am I bid, what am I bid?"

According to the news channel, the victims of the slave trade were migrants from elsewhere in Africa hoping to cross the Mediterranean with the help of people smugglers, and migrate to Europe illegally.

CNN slave Libya investigation slave seller

Foto: A man whom CNN say was conducting the auction. source CNN

However, increasing efforts by European and Libyan officials to stop the crossings means that increasing numbers of men are getting stuck in Libya with nowhere to go, CNN said.

When they run out of money to pay the traffickers, they are taken as slaves and sold for farm work. At a migrant camp on the Libyan coast, several men told CNN they have been enslaved as well, and were beaten and abused during the process.

Libyan authorities promised to investigate, CNN said. Here is a video report showing more of the investigation:

A press release from CNN said more would be aired on Tuesday's episode of "Amanpour," shown on CNN International at 7 p.m. GMT and 2 p.m. ET.