The United Nations' mission in Libya voiced alarm Thursday at reports that dozens of detainees at a prison near the capital Tripoli are on hunger strike.

"UNSMIL is alarmed by reports that more than 70 inmates at Mitiga Central Prison are on a hunger strike," it said in a statement.

They had launched the strike last month to protest "the prolonged arbitrary detention of many inmates, conditions of detention and ill treatment, including denial of family visits and medical care," it said.

It also cited reports that treatment of the hunger strikers had been worsened in "retaliation."

"UNSMIL calls on judicial authorities to investigate these reports, immediately release all persons detained without legal basis and ensure the rights of all detainees in line with Libya's international obligations and national laws," it said.

The prison is on an airbase that also includes the capital's only operational airport.

It is controlled by the powerful Al-Radaa force which acts as the police in Tripoli, often arresting both suspected jihadists and ordinary criminals.

The force has been criticized by rights groups for its treatment of detainees.

Libya has been mired in chaos since the 2011 revolt that overthrew dictator Moammar Gadhafi, with a broad array of militias and foreign groups vying for power.