Palestinian factions in Syria's Yarmouk refugee camp have agreed to join forces with the Syrian government in their fight against Islamic State militants who have invaded the camp.
Islamic State fighters overran much of Yarmouk last week, establishing a foothold in the Syrian capital Damascus for the first time.
The camp itself had already been under siege by Syrian government forces for two years before the Islamic State moved in.
Anti-government Palestinian militias and members of the Free Syrian Army have been trying to hold off the Islamic State fighters.
The new military alliance is backed by the Palestine Liberation Organization.
UN Secretary General Ban Ki-moon is warning the new alliance will only worsen the humanitarian situation in Yarmouk.
He is calling for an immediate ceasefire.
"After more than two years of a merciless siege, 18,000 Palestine refugees and Syrians are now being held hostage by Da'esh and other extremist militants. A refugee camp is beginning to resemble a death camp. The residents of Yarmouk - including 3,500 children -- are being turned into human shields. They face a double-edged sword - armed elements inside the camp, and government forces outside. We are now hearing worrying reports of a massive assault on the camp and all civilians in it. This would be yet one more outrageous war crime for which those responsible must be held accountable."
Syria's bloody conflict, which has entered its fifth year, has claimed the lives of more than 200 thousand people.
Millions of others have been forced from their homes.
