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BEIRUT, Lebanon — ISIS fighters took control of large parts of a besieged Palestinian refugee camp in Syria’s capital on Wednesday, witnesses and a monitoring group said.

The Yarmouk camp, home to up to 18,000 people, has been caught between government forces and Syrian insurgent groups including ISIS's rivals including al Qaeda's Nusra Front.

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"They pushed from the Hajar Aswad area and Nusra fighters have joined them. They have pledged loyalty to Daesh," one witness said, using an Arabic term for ISIS.

The witness and the British-based Syrian Observatory for Human Rights said clashes continued inside the devastated camp where food, medicine and water are scarce.

The Observatory said ISIS controlled some of the main streets in the camp.

Chris Gunness, a spokesman for the United Nations Relief and Works Agency, said the group is "extremely concerned" about the safety of Syrian and Palestinian civilians in the Yarmouk camp in Damascus. The agency is calling for an end to the fighting.

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— Reuters and NBC News