The UN decried Saturday "reliable reports" that smugglers and traffickers were impersonating its staff in Libya to target desperate migrants and refugees, and demanded action to hold them to account.
The UN refugee agency said reliable sources were indicating that smugglers and traffickers trying to pretend they worked with different UN organisations had been seen in a range of different locations across the conflict-torn country.
"These criminals were spotted at disembarkation points and smuggling hubs, using vests and other items with logos similar to that of UNHCR," it said in a statement.
The agency said it had received information about the problem from among other "refugees who report having been sold to traffickers in Libya, and subjected to abuse and torture, including after having been intercepted at sea".
Spokesman Babar Baloch however told AFP that it remained unclear if the fake UN employees had succeeded in tricking migrants and refugees who subsequently faced abuse, saying the agency was still investigating.
"We want the authorities to go after these people," he said, stressing that they clearly had "criminal intent" and were going after people who were already extremely vulnerable.
Plunged into chaos following the fall of longtime dictator Moamer Kadhafi in a 2011 NATO-backed uprising, Libya has become a prime transit point for sub-Saharan African migrants making dangerous clandestine bids to reach Europe.
People smugglers have taken advantage of the turmoil, putting African migrants seeking to reach Europe at greater risk.
Many migrants, intercepted or rescued at sea, find themselves detained in detention centres with poor conditions.
UNHCR said Saturday that it and its partners are present at all the official disembarkation points "to provide lifesaving humanitarian and medical assistance".
"Once the smuggled passengers are back on land, the Libyan authorities transport them to detention centres, managed by the Directorate for Combatting Illegal Migration," the agency said, adding that its staff do not help with such transports but they are present at the centres to monitor the situation and provide assistance.
UNHCR pointed out that the situation for refugees and migrants in the Tripoli area has significantly worsened in recent weeks amid heavy clashes.
Last week, the agency helped evacuate some 300 migrants from the Ain Zara detention centre to another a few kilometres away as they risked getting caught in the crossfire.
Aid group Doctors Without Borders has meanwhile urged the evacuation of thousands of migrants trapped in detention centres in Tripoli, where a fragile ceasefire was negotiated last week.
But those in detention centres are not the only ones at risk.
UNHCR said Saturday it had received reports of "unspeakable atrocities" committed against migrants in the streets of the Libyan capital, including rape, kidnapping and torture.