The UN refugee agency was looking into possible violations of international law by an Italian-flagged mercantile ship, which rescued more than 100 migrants and returned them to Libya earlier this week.
The UN High Commissioner for Refugees said on Twitter that Libya is not safe and the operation "could represent a violation of international law."
Spanish charity Proactiva Open Arms said their boat had heard radio communications between the Italian ship and the Libyan authorities that confirmed that the 108 migrants were rescued from international waters on Monday and taken back to Libya, their country of departure.
The UN agency is investigating whether the rescue operation took place in international waters or in Libyan waters, as claimed by the Italian coast guard.
International law states that migrants rescued in international waters cannot be returned to a place where their lives are put in danger. Libya is classified as unsafe by both the United Nations and European Union.
Italian Interior Minister Matteo Salvini said on Facebook on Tuesday that the Italian coast guard was not involved in the rescue and that it was coordinated by the Libyan coast guard.
Salvini, who heads Italy's far-right League party, has vowed to stop migrants from arriving on Italian coasts. He has refused permission to NGO and charity rescue boats operating in the Mediterranean to dock in Italy, accusing them of aiding human traffickers.
ap/kms (AP, Reuters)