(CNN) More than 100 migrants died when their boat sank off the Libyan coast at the beginning of September, and the survivors are being detained in that country, according to the French charity Medecins Sans Frontieres (MSF).
MSF said two rubber boats left Libya early on September 1, carrying hundreds of people from countries including Sudan, Mali, Nigeria, Cameroon, Ghana, Libya, Algeria and Egypt. But one of the boats with 165 adults and 20 children on board deflated and sank, leading to dozens of deaths, one eyewitness told MSF.
The Libyan Coast Guard took 276 survivors to the port city of Khoms and only two bodies were reportedly recovered. MSF provided urgent medical assistance after their disembarkation.
"We called the Italian coast guard and sent our coordinates, asking for assistance as people started to fall in the water," one survivor said.
"We were told they would send someone. But the boat started sinking. We couldn't swim and only a few people had life jackets. Those among us who could hold on to the boat's floating hood stayed alive. [European] rescuers came later by air and threw life rafts but everybody was in the water; the boat had already sunk."
Libyan rescuers eventually arrived, but only saved 55 people.
An MSF team treated some survivors for chemical burns caused by contact with spilled engine fuel.
The survivors were transferred to a detention center in the north African nation where MSF says it has provided further care but is concerned about their safety. Among the detained are pregnant women, children and infants, and people with serious medical conditions and chemical burns.
"How can they recover when locked inside cells, in very poor hygiene conditions and sleeping on blankets or mattresses placed on the floor that cause incredible pain for those suffering from severe burns?" said Jai Defrancis, an MSF nurse working in the Libyan coastal city of Misrata.
"Some of them cannot even sit or walk. We have started to see patients with severe chest infections like pneumonia, caused by being in the water for such a long time."
The news follows a warning on Monday by the UNHCR, the UN's refugee agency, that human traffickers in Libya are gaining access to migrants by posing as aid workers.
"Reliable sources and refugees have reported criminals using vests and other items with logos similar to that of UNHCR, at disembarkation points and smuggling hubs," the UNHCR said in a statement.
UNHCR is opposed to the detention of refugees and migrants, but has staff monitoring the situation at Libyan detention centres, aiding and identifying the most vulnerable.
However, the agency insists it does not engage in the transfer of refugees from disembarkation points to detention centers.
"The reports of criminals impersonating UNHCR staff come as the situation for refugees and migrants detained or living in the Libyan capital, Tripoli, has dramatically deteriorated," the UN said.
Violent clashes in Tripoli that erupted on August 26, when tanks and heavy artillery were deployed in residential neighborhoods, has led to atrocities committed against refugees and migrants, including rape, kidnapping and torture, the UN said.
More than 1,500 migrants have died while attempting to cross the Mediterranean this year, according to the International Organization for Migration (IOM). With the support of the European Union, the Libyan Coast Guard returned 13,185 refugees and migrants to Libya between January and August 2018.
MSF on Monday called on the UNHCR and safe countries to "rapidly organize the evacuation of refugees and asylum-seekers from Libya and expedite their resettlement," including helping those who want to return to their country of origin.