The United Nations agency for Palestinian refugees, UNRWA, said it was deeply concerned about the lack of humanitarian access to the civilians inside Yarmouk, strategically located on the southern tip of Damascus.

For more than a week now, Yarmouk, Syria's largest refugee camp, has been under fire from ISIS terrorist.

ISIS overran the camp on April 9 and new fighting has erupted since the early hours of Friday on the edge of the camp, which is close to the area of Hajar al-Aswad where ISIS has bases already, said Khaled Abdel-Majid, the secretary general of the united Palestinian factions that are fighting the extremists.

"Our force is now in the center of the camp after confining Islamic State terrorists to only 35 per cent of the camp," he told dpa by phone.

Ayman Abu Hashim, who leads a Palestinian refugee committee in the Syrian opposition in what called "interim government", said that "Islamic State" insurgents had retreated from about 70 per cent of the camp.

He told dpa that before its withdrawal, the al-Qaeda splinter militia handed over its posts inside Yarmouk to the al-Qaeda-linked al-Nusra Front.

Palestinian factions in Damascus have agreed to form a united force backing the Syrian army to drive the extremists out of Yarmouk.

Humanitarian groups have warned of a dire situation in Yarmouk, where at least 14,000 civilians are trapped.