More than 100 migrants died in a shipwreck off the Libyan coastline earlier this month.
According to an aid agency, Medecins Sans Frontieres MSF, two rubber boats set off on September 1, but one of the vessels deflated and sank.
The 276 survivors were eventually taken to the Libyan port city of Khoms, around 100km 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.
According to the International Organization for Migration IOM, so far, more than 1,500 migrants have died while attempting to cross the Mediterranean this year,