BEIRUT — Palestinian fighters clashed with Islamic State militants in a heavily contested Palestinian refugee camp in the Syrian capital on Monday as a United Nations official described the situation in the embattled camp as “beyond inhumane.” The heavy clashes have added yet another layer of misery for the nearly 18,000 Yarmouk residents who have already endured desperate conditions marked by a lack of basic food, medicine and water. After heavy fighting on Sunday, sporadic clashes broke out on Monday in Yarmouk, according to Hatem al-Dimashqi, an activist based in an area just south of Damascus, and the Syrian Observatory for Human Rights. The fighting inside the camp has largely pitted the Islamic State group against Aknaf Beit al-Maqdis, a Palestinian faction opposed to Syrian President Bashar Assad. Chris Gunness, a spokesman for the U.N.