Fighting in the Damascus district of Yarmouk have interrupted aid distribution to thousands of besieged Palestinian refugees, the United Nations said on Monday.The clashes broke out on Sunday in Yarmouk, where 20,000 people have been trapped for months by Syria's civil war and are dependent on humanitarian supplies delivered by UNRWA, the UN agency which supports Palestinian refugees.The Syrian Observatory for Human Rights, a Britain-based monitoring group, said an ambulance driver was killed in mortar fire on Sunday and residents reported several explosions.A Palestinian group that supports President Bashar Assad, the Popular Front for the Liberation of Palestine-General Command, blamed the fighting on al-Qaida's Syrian branch, the Nusra Front.It said "terrorist groups from the Nusra Front and their takfiri brothers... infiltrated Yarmouk Camp" on Sunday. A "takfiri" denotes someone who brands others, including some Muslims, as infidels, often as justification for fighting and killing them. UNRWA spokesman Chris Gunness said the UN agency was unable to distribute food parcels in Yarmouk on Sunday and called on all sides in the conflict to immediately allow the resumption of the aid operation. "UNRWA remains deeply concerned about the desperate humanitarian situation in Yarmouk and the fact that increasing tensions and resort to armed force have disrupted its efforts to alleviate the desperate plight of civilians," he said.Yarmouk is one of several besieged districts in Damascus and other cities. Most areas have been cut off by Assad's forces who are trying to force the mostly Sunni Muslim rebels into submission, or at least turn civilians against them.But insurgents are also surrounding two Shi'ite Muslim towns in the northern province of Aleppo. Assad is an Alawite, a sect affiliated with Shi'ite Islam.Local ceasefires in Yarmouk and other Damascus suburbs, as well as the central city of Homs, have allowed some civilians to escape and limited amounts of food to be taken in, but the deals are fragile and in some cases the sieges have resumed.The United Nations Security Council called last month for the immediate lifting of all the sieges and demanded all sides allow access for humanitarian aid.