More than 100 migrants died in a shipwreck off the Libyan coastline earlier this month, an aid agency says.
Two rubber boats set off on 1 September but one of the vessels deflated and sank, Medecins Sans Frontieres (MSF) says.
The 276 survivors were eventually taken to the Libyan port city of Khoms, around 100 km (62 miles) to the south-east of the capital Tripoli.
MSF says the group is now being held in “arbitrary detention”.
The survivors, including pregnant women, children and infants, have been treated by MSF for pneumonia or burns from leaked fuel.
It appears to be the largest migrant death toll in the Mediterranean in several months.
In early June, 112 people reportedly drowned after a boat sank off the Tunisian coast. Later the same month, the UNHCR said that 220 people had died in three separate incidents over the course of two days.