Around 15,000 African migrants will be repatriated from Libya in the next two months under an emergency plan to stop abuses there, EU Foreign Policy Chief Federica Mogherini said on Thursday.

European and African leaders announced the plan for accelerated "voluntary" deportations at a summit in Abidjan two weeks ago but did not set a timetable for repatriating those left in Libyan government detention centres.

Mogherini, ahead of an EU summit, said the repatriations should be done by February.

"In only two months we hope and we expect to manage to assist the voluntary return of 15,000 people," she told reporters in Brussels.

The European Union (EU) and the African Union (AU) are working with the UN International Organisation for Migration, and the Libyan authorities to arrange for the returns to their home countries in sub-Saharan Africa.

In the last year, 16,000 people have returned home, but EU and AU leaders agreed in Abidjan to accelerate returns following CNN television footage of a slave market in Libya, where smugglers and criminal networks act with impunity.

It provoked an international furore on top of reports of rape, torture and beatings of migrants in Libya, including in detention camps under the control of the UN-backed government of Libyan Prime Minister Fayez Al Sarraj.

The EU faced accusations from rights groups that it created conditions for such abuses via its cooperation with Sarraj to detain migrants using Libya as a jumping board to Italy, the main entry point to Europe.

EU, UN and AU officials all admit it wil be a major challenge to repatriate or protect the 700,000 migrants in Libya, most of whom are in areas outside government control.

Amira El Fadil, the AU's commissioner for social affairs, told the press conference the key was finding a political solution to the chaos and violence in Libya.

Mogherini, a former Italian foreign minister, said 2,000 migrants have been returned to their homes since the Abidjan summit. 

The EU's top diplomat also announced another 100 million euros for the multi-billion euro Africa trust fund, with the money to finance flights from Libya and to help the migrants resettle.

Mogherini said she would also ask EU member states at a summit dinner to contribute more to the trust fund.

Four eastern EU countries which have triggered criticism for refusing to admit refugees — Hungary, Poland, the Czech Republic and Poland — announced 35 million euros for the trust fund to bolster the EU's external borders and Libya's frontiers.