The EU said Thursday it is working without "letup" for a durable solution to the plight of migrants in Libya, adding it shared French President Emmanuel Macron's anger over slave markets there.

The US network CNN triggered a wave of condemnation when it aired footage last week of an apparent auction where black men were presented to North African buyers as potential farmhands and sold.

"I fully share the French president's indignation," European Union migration commissioner Dimitris Avramopoulos told AFP after Macron called the slave auctions a "crime against humanity."

"We are all conscious of the appalling and degrading conditions in which some migrants are held in Libya. This cannot last," Avramopoulos said in an email exchange with AFP.

"It is exactly for this reason that the European Union is working without letup, on all fronts, with its international partners to find durable solutions," he added.

Such solutions, he said, must comply with founding EU values of solidarity and respect for human rights.

But the European Union -- where Macron is trying to carve out his influence -- has also been criticised for cooperating with the Libyan coastguard in seeking to block migrants from leaving.

The UN this month deplored an EU policy of helping the Libyan authorities intercept migrants trying to cross the Mediterranean and return them to "horrific" prisons in Libya.

Guinean president Conde on Wednesday also criticised Europe's cooperation with the Libyans.

"The refugees are living in extremely bad conditions," he said. "Our European friends were not right to ask Libya to keep immigrants (in detention)."

The AU leader said he was "in contact with all leaders", adding: "We are trying to find a solution, even if it means bringing all of our citizens home."

UN human rights chief Zeid Ra'ad Al Hussein this month accused the international community of turning a "blind eye to the unimaginable horrors endured by migrants in Libya" and called the EU's policy "inhuman".

EU foreign policy chief Federica Mogherini said in Brussels on Wednesday that UN agencies in Libya backed by EU aid in Libya have helped some 10,000 stranded migrants return voluntarily to their countries of origin.

"I hope that our work, which started already a year ago, will be reinforced by this new wakeup call over the gravity of the situation on the ground," she said as she hosted talks with a senior AU official.