More than 12,000 detainees are being held officially in 27 prisons and detention centres across Libya, the UN has said in a new report.

According to MENA news agency, thousands more are held illegally and often in “inhumane conditions in centres controlled by armed groups or "secret" bases, Secretary General Antonio Guterres said in the report to the UN Security Council, which was obtained on Monday, according to an AP report.

Oil-rich Libya has been engulfed in chaos since a Nato-backed uprising toppled and killed longtime dictator Muammar Qaddafi in 2011. The North African country has, in recent years, emerged as the dominant transit point for migrants fleeing war and poverty in Africa and the Middle East, hoping for a better life in Europe.

Traffickers have exploited the crisis and often pack desperate families into ill-equipped rubber or wooden boats that stall and founder along the perilous Central Mediterranean route.

The UN political mission in Libya, known as UNSMIL, continues to document cases of arbitrary detention, torture, sexual violence and other breaches of international law in centres operated by the government and other groups, Mr Guterres said.

“I remain gravely concerned by the continuing violations of the human rights of migrants, refugees and asylum seekers in Libya,” Mr Guterres said.

“Female and male migrants and refugees continued to face heightened risks of rape, sexual harassment and trafficking by armed groups, transnational smuggles and traffickers, as well as officials from the Directorate for Combating Illegal Migration, which operates under the Ministry of Interior,” he said.

Mr Guterres said the widespread arbitrary detention of migrants and refugees continued, including those rescued or intercepted trying to cross the Mediterranean Sea to Europe and returned to Libya by the Libyan coastguard.

As of December 14, he said, the coastguard had intercepted 30,990 migrants and refugees and returned them to Libya, “almost three times the total number of people returned in 2020”.

More than 1,300 people have died or disappeared attempting the journey, he said.