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.
Eyewitnesses and media reports said fighters had entered the camp from the neighbouring suburb of Hajar Aswad.
IS “now controls large parts of the camp”, the UK-based Syrian Observatory for Human Rights, a monitoring group, said.
An official with the Palestine Liberation Organisation, based in Damascus, also told AFP that “the majority of the camp” had been seized by IS.
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.”