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.
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.
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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.
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.
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.
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