A leading humanitarian organization is asking the Libyan government for more transparency about the fate of migrants returned to the country after being stopped at sea.

Libya watchers have noted a decrease in the number of boats with migrants leaving the country over the past approximately six weeks and at the same noticed an uptick in numbers returned.

But concerns are growing over what happens to migrants returned to Libya, prompting Doctors Without Borders, also known as Medecins sans frontieres (MSF), to call for greater transparency.

"With no rule of law in Libya, the detention system is unregulated," Annemarie Loof, MSF's operational manager for migration, told Maghreb News Wire. "New detention centers emerge overnight and shut down just as suddenly, the fate of people detained there unknown."

She added, "People are transferred between different detention centers, moved to undisclosed locations or disappear overnight. Detainees do not know if and when the detention will end and have virtually no access to the outside world."

MSF has asked for transparency about the fate of people returned to Libya, including more information on where the people are taken and what happens in the detention center system, Loof said.

"The reasons for the slow decrease in the number of boats coming out of Libya the last few weeks aren’t entirely clear," she said.

But it is clear, she said, the support of the EU and deployment of the Italian Navy is part of a "broader strategy to seal off the coast of Libya and keep refugees, migrants and asylum-seekers 'contained' in a country where they are exposed to an alarming level of violence and exploitation."

Libya-based sources of Italian newspaper Il Giornale suggest the key to the reduction in the number of migrants leaving the Libyan coastline is an agreement between Faiez Serraj’s government and local tribes.

According to sources quoted by Il Giornale, the Italian government is funneling funds and aid to the local tribes that control smuggling routes. Those sources allege the Italian Navy's mission is a cover for this agreement.

Italian Defense Minister Roberta Pinotti said the country is providing "logistical, technical and operational support for Libyan naval vessels," according to a Reuters report.

MSF argues that despite the slowdown in numbers, there is still a need for a presence on the sea and that the ship, Aqaurius, operated by the organization with SOS Mediterranee, has rescued hundreds of people in recent weeks.

"This included the recovery of eight dead people who had asphyxiated in fuel and water at the bottom of a rubber boat – another reminder of harsh reality we see on the Mediterranean," Loof said.

MSF provides health care to refugees, asylum-seekers and migrants at seven detention centers in Tripoli. Others are inaccessible or run by different organizations.

“No one seems to know exactly what happens to people who are returned by the Libyan Coast Guard and disembarked in Libya," Loof said. "It is impossible to track whether people are brought to detention centers, and if they are, to which ones. There doesn’t seem to be a registration system or any judicial oversight and therefore there is no accountability."