Palestinian factions in the Yarmouk refugee camp in Damascus have agreed to action by Syrian government forces to combat Islamic State (IS), Palestinian officials say.
The situation has deteriorated since 1 April, when IS launched an assault.
Before that the camp had been under siege by government forces for months, creating a grave humanitarian crisis.
It is unclear whether all Palestinian factions in the camp have agreed to Syrian government intervention.
Anti-government Palestinian militiamen from Aknaf Beit al-Maqdis and some Free Syrian Army fighters have been leading the fight against IS.
"The operation will be conducted in co-operation between the Palestinian groups in Syria and the Syrian government through a joint operation centre," Palestine Liberation Organisation (PLO) official Ahmed Majdalani - who is heading a Palestinian delegation in Damascus - told the BBC.
Who exactly will do what in the operation, and how the plan will work, is far from clear, the BBC's Jim Muir reports from Beirut.
Aknaf Beit al-Maqdis was not involved in the agreement - it has been co-operating with Syrian rebels battling both government forces and Islamic State, our correspondent reports.
If the new agreement ends up seeing Syrian troops moving into the camp with the PLO's blessing, the government is likely to be accused by some of manipulating the situation to take control of a highly strategic area on the approaches to the city centre, he adds.
On Wednesday, the UN Security Council demanded humanitarian access to the camp, with one UN official describing the situation for the 18,000 refugees there as "beyond inhumane".
The Palestinian ambassador to the UN, Riyad Mansour, has appealed to member nations to relocate the refugees elsewhere in Syria or abroad.
A few hundred people were reported to have managed to escape the camp over the weekend.
Monitors say IS and the al-Qaeda-affiliated al-Nusra Front, who have fought each other in other parts of Syria, are working together in Yarmouk.
Syria's bloody conflict, which has entered its fifth year, has claimed the lives of more than 200,000 Syrians.
The battle between forces loyal to President Bashar al-Assad, rebels opposed to his rule and Islamic State has also driven more than 11 million people from their homes.
Yarmouk was first built for Palestinians fleeing the 1948 Arab-Israeli war.
Before the Syrian civil war began, it had more than 150,000 refugees living there and had its own mosques, schools and public buildings. But since 2012, the camp has been besieged by fighting.