ISIS has taken control of 90 percent of a Palestinian refugee camp on the Damascus outskirts where 18,000 refugee live, a monitoring group said on Saturday.

The hardline group's offensive in Yarmouk as most powerful insurgent group cause heavy clashes with Palestinian and other rivals, Reuters reports.

The United Nations has said it is extremely concerned about the safety and protection of Syrians and Palestinians in the camp. Civilians trapped there have long suffered by terrorist group.

"Yarmouk is a test, a challenge for the international community. We must not fail. The credibility of the international system itself is at stake," U.N. Relief and Works Agency (UNRWA) spokesman Chris Gunness said.

ISIS on Wednesday launched an attack on other groups of fighters in Yarmouk, in particular Aknaf Beit al-Maqdis, an anti-government militia of Syrians and Palestinians from the camp.

ISIS supporters posted photos on social media of the severed heads of two men they said had been beheaded after fighting for Aknaf Beit al-Maqdis.

The Syrian Observatory for Human Rights, an antigovernment group which monitors the conflict from Britain, said ISIS and al Qaeda's official Syria wing, the Nusra Front, made gains overnight, pushing into the northeast of the district, close to central Damascus. They now control 90 percent of the camp, it said.

Syrian Observatory for Human Rights als report that 40 isis militants killed in clashes with armed Palestinian refugees inside the Yarmouk camp near the Syrian capital, Damascus.