The aid group Doctors Without Borders (MSF) on Friday urged the evacuation of thousands of migrants trapped in detention centers in Libya's capital Tripoli, which is under a fragile ceasefire after deadly clashes.

The ceasefire brokered by the United Nations was agreed between armed groups Tuesday after at least 63 people were killed in days of fighting around the city.

Doctors Without Borders said migrants in squalid detention centers close to the frontline could yet come under shelling and artillery fire. Analysts say the ceasefire is doomed without a sustainable political agreement between the myriad armed groups. Ibrahim Younis, MSF's mission chief in Libya, said food supplies to the detention centers were sparse as international groups had been forced to suspend their work due to the clashes. The charity has been completely cut off from one of the most crowded detention centers, depriving some 700 people of aid, and has pulled some staff from the city.

The U.N.'s refugee agency, the UNHCR, last week said it had "evacuated" some 300 migrants from the Ain Zara detention center to another a few kilometers away as they risked getting caught in the crossfire. But Younis said this was not good enough. The aid group called on the UNHCR and the governments of peaceful countries to organize a mass evacuation for the migrants, who could then claim asylum.