More than 200 migrants found in small boats off Malta have been sent back to Libya.
It is reported this is the first time that Tripoli has accepted the transfer of migrants under a previous accord.
Italian police and coastguard boats are said to be involved in the transfer.
Rome says Tripoli’s acceptance of the migrants could mark a major step forward in the effort to stop Africans trying to reach Europe by boat, often from Libya.
However, the UN refugee agency is concerned that some of those sent back to Libya might have been genuine asylum seekers.
UNHCR spokesman William Spindler said: “This incident marks a significant shift in policy by the Italian government and it’s a source of very serious concern. The UNHCR deeply regrets the lack of transparency which surrounded the event.”
The UNHCR is calling on Italy and Malta to ensure that people picked up at sea, demanding asylum protection, are given access to what it calls “territory and procedures”.
The latest case follows a standoff last month over who would take in some 140 migrants picked up by a Turkish ship off Malta.