Civilians fleeing the DAESH offensive in northwestern Syria have been prevented by the Democratic Union Party's (PYD) People's Protection Units (YPG) militia from entering areas under their control in response to rebel shelling of a YPG-held area of Aleppo, the United Nations said. Civilians have been fleeing the fighting between the YPG and DAESH, which advanced into the opposition-held town of Marea over the weekend.The United Nations has expressed concern about thousands of Syrians trapped by fighting in northern Aleppo.All the main parties to the Syrian civil war are fighting in the Aleppo area, and the rebels who are fighting DAESH north of the city have also been involved in hostilities with the YPG, which controls wide areas of territory to the west of Marea.A situation update from the U.N. Office for the Coordination of Humanitarian Affairs (OCHA) said about 2,000 people had managed to evacuate Marea and nearby Sheikh Issa, which DAESH encircled on May 27."However, an estimated 7,000 civilians still remain inside and unable to leave due to restrictions imposed by Kurdish authorities," it added, saying it was a Kurdish response to the rebel shelling of Aleppo's Kurdish-held Sheikh Maqsoud area.Meanwhile, opposition authorities in the rebel-held town of Azaz near the Turkish border have issued a directive to not let in any more people fleeing DAESH-held areas.The court that issued the order cited fears of infiltration by covert DAESH militants posing as internally displaced people (IDPs), after 8,000 IDPs arrived in Azaz, the OCHA said.Pablo Marco, the regional manager of Doctors Without Borders, said concerns were rising for a large civilian population less than 5 kilometers from advancing DAESH militants."We are talking about 100,000 people who are trapped a few kilometers from ISIS [DAESH]. They are terrified, there is nowhere to go," Marco said in a telephone interview with Agence France-Presse (AFP).DAESH swept toward the last rebel strongholds of Marea and Azaz in Aleppo province on Friday, forcing thousands to flee toward the northern border.