Islamic State (IS) militants have entered the Palestinian refugee camp Yarmouk in Damascus, activists and Palestinian officials say.Clashes erupted between the militants and groups inside the camp, with IS seizing control of large parts of the camp, reports said.
The UN says about 18,000 Palestinian refugees are inside the camp.
IS militants have seized large swathes of territory in eastern Syria and across northern and western Iraq.
Yarmouk residents told BBC Arabic that members of Aknaf Beit al-Maqdis, a group formed by Palestinian militiamen opposed to the Syrian government, were leading the fight against the IS militants, along with some Free Syrian Army fighters.
IS fighters had seized control of large parts of the camp, an official with the Palestine Liberation Organisation based in Damascus, and the UK-based Syrian Observatory for Human Rights, said.
There has been no official statement from IS about the move.
Members of the self-proclaimed Islamic State stormed into the southern side of Yarmouk camp in the early hours of the morning and clashed with the Palestinian brigade, Aknaf Beit al-Maqdis.
Reports suggest they came in from the area of Hajar al-Aswad in the south of the capital. There has been no evidence in the past that IS had any foothold in Damascus.
The Palestinian Ambassador to Damascus, Anwar Abdulhadi, told the BBC that the group had seized the area of the camp near the Palestine Hospital.
Most information is coming from Palestinian officials in areas under government control.
The attack comes days before a deal to ease the humanitarian situation for civilians in the camp was set to come into operation.
Extreme hardship'First built for Palestinians fleeing the 1948 Arab-Israeli war, Yarmouk was once considered by many to be the de facto capital of the Palestinian refugee diaspora.
Prior to the Syrian civil war, it had more than 150,000 refugees living there, and its own mosques, schools and public buildings.
However, the camp has been besieged by fighting between government troops and rebel forces since 2012.
Unrwa, a UN relief agency for Palestinian refugees, says about 18,000 refugees remain trapped in the camp, with inadequate access to food supplies, clean water and electricity.
In March, Unrwa said: "The extreme hardship faced by Palestine refugees in Yarmouk, but also in other locations in Syria as a result of the armed conflict is, from a human point of view, unacceptable."