The United Nations mission in Libya condemned Sunday the "heinous killing" of 15 migrants near the Mediterranean coastal city of Sabratha, accusing smugglers and demanding justice.

The bodies were found on a beach on Friday morning, most of them burned inside a charred boat, according to the UN and the Libyan Red Crescent.

"While the exact circumstances remain to be determined, the killings reportedly resulted from clashes between rival traffickers," the UN mission UNSMIL said in a statement.

It urged authorities in the North African nation "to ensure a swift, independent and transparent investigation to bring all perpetrators to justice".

Libya was a key route for clandestine migration even before the 2011 uprising that overthrew dictator Moamer Kadhafi.

The lawlessness that ensued bolstered its position on the world's deadliest migration route across the Mediterranean to Europe.

People smugglers from the western city of Sabratha -- just 300 kilometres (185 miles) from the Italian island of Lampedusa -- continue to play a key role.

Migrants often face horrific treatment at the hands of smuggling gangs.

Rights groups have repeatedly accused authorities and armed groups operating under state auspices of torture and other abuses.

The latest killings are "a stark reminder of the lack of protection migrants and asylum seekers face in Libya, and the widespread human rights violations undertaken by powerful trafficking and criminal networks who need to be swiftly stopped and prosecuted," UNSMIL said.

Libyan media reported that the killings resulted from a "dispute between people smugglers" that led to them opening fire on the migrants, mostly from African countries further south.

One of the groups involved set fire to the boat, according to the reports.

Since the start of the year, more than 14,000 migrants have been intercepted and returned to Libya, the International Organization for Migration said Monday.

At least 216 people have died attempting to cross the Mediterranean Sea and 724 are missing and presumed dead.