The Daesh group militants clashed Saturday with US-backed fighters in Syria's Manbij as a 48-hour deadline loomed for them to leave the battleground town, a monitor said. The ultimatum was issued Thursday by the Syrian Democratic Forces (SDF), an Arab-Kurdish alliance that is fighting Daesh with support from a US-led coalition. The militants are accused of using civilians as human shields in Manbij, located in the northern province of Aleppo on IS's main supply route between Syria and Turkey. The ultimatum came after the Syrian Observatory for Human Rights, a British-based monitor, said that at least 56 civilians, including children, were killed on Tuesday in coalition air strikes near Manbij. Coalition spokesman Colonel Chris Garver said on Friday that Daesh had "used civilians as human shields and as bait" in order to draw the fire of the SDF towards civilians. Observatory chief Rami Abdel Rahman said there were sporadic clashes inside Manbij and raids by the US-led coalition on the town as the deadline approached for Daesh to leave. "Daesh is fiercely resisting attempts by the SDF forces to advance inside the city and is pushing children towards the frontlines in spite of the deadline," he said. The coalition spokesman said that the militants were mounting an exceptionally tough fightback in Manbij. Fighting has grown more intense as SDF units move into the city, he said, "which is sort of different than what we saw in Ramadi and what we saw in Fallujah," two Iraqi cities from which militants were ousted this year. "It's a fight like we haven't seen before," said Garver. He estimated that the SDF had taken back roughly half the city, an area still housing at least 2,000 civilians. Garver said he could not confirm that the SDF had issued an ultimatum to Daesh fighters to leave Manbij. He said that Tuesday's air raid was called after the SDF "observed a large group of Daesh fighters in a convoy who appeared to be readying for a counterattack." "The strike was against both buildings and vehicles," said Garver. Afterwards, the spokesman said, the coalition received both internal and external reports "that there may have been civilians in the area who are mixed in and among the Daesh fighters." The US-led coalition has opened an investigation into the reports of civilian deaths, which have sparked condemnation including from Syrian activists and opposition groups.