The jihadists 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 48-hour period is over, and there will be no more opportunities like this one for Daesh (IS)," a Manbij Military Council commander told AFP on condition of anonymity.
IS has "not responded" to the SDF's offer and had instead "attacked our positions" in Manbij, he said.
"We will intensify our attacks on their remaining positions in the town," the commander said, adding that the SDF was working to "secure safe passages" for civilians in Manbij looking to flee.
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 IS 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 IS to leave.
"IS 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 jihadists 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 jihadists 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 IS fighters to leave Manbij.
He said that Tuesday's air raid was called after the SDF "observed a large group of Daesh (IS) 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 (IS) 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.
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"The 48-hour period is over, and there will be no more opportunities like this one for Daesh (IS)," a Manbij Military Council commander told AFP on condition of anonymity.
IS has "not responded" to the SDF's offer and had instead "attacked our positions" in Manbij, he said.
"We will intensify our attacks on their remaining positions in the town," the commander said, adding that the SDF was working to "secure safe passages" for civilians in Manbij looking to flee.
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 IS 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 IS to leave.
"IS 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 jihadists 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 jihadists 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 IS fighters to leave Manbij.
He said that Tuesday's air raid was called after the SDF "observed a large group of Daesh (IS) 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 (IS) 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.
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