In the past two months, more than 37,000 people, mostly wives and children of jihadist fighters, have fled into SDF-held areas, the Observatory says.
"The mostly foreign fighters were put in seven trucks and taken away" by the USA -led coalition and the US -backed Syrian Democratic Forces, Abdurrahman said. "If we managed successfully to isolate civilians, we are talking about few days to finish the battle, but if terrorists continued to use civilians as human shields, it might take more than just a few days".
It said the group rejects to surrender to the US -backed SDF.
Several people identified themselves as Iraqi, Turkish, Ukranian or Russian. He said the women and children coming out are treated as civilians "even if they are families of Daesh".
"We are seeing fierce resistance from the Daesh fighters", SDF field commander Adnan Afrin told Reuters on the outskirts of Baghouz. Other European countries have remained largely silent about the fate of men and women whom many see as a security threat. "We can hear them on the walky-talkies".
"The bombing was unimaginable, we ran from one place to the other", said Hala Hassan, 29, a woman from Syria's Deir al-Zor who escaped with her five children.
Additionally, he said those remaining in the province are professional terrorists from countries all over the world, who have worked with groups like Al Qaeda. "There was no food".
"Most of those who got out are foreigners", said Abdel Rahman.
She said her husband had moved the family across the border into Syria almost three years ago and she did not leave the enclave earlier because of the shelling.
The official, speaking on condition of anonymity, said the USA has identified about 50 people among the more than 900 held by Syrian forces as "high value" suspects that could be transported to Guantanamo if they are not repatriated. "ISIS fighters continue to conduct counter attacks", he said by email on Tuesday.
Coalition air power, crucial to the SDF's advances, has leveled entire districts of towns and cities in the fight against Islamic State, though it says it takes care to avoid hitting civilians.
A UK-based war monitor and Syrian state media said strikes had killed 16 civilians overnight in a camp in Baghouz.
Ryan said the coalition was aware of the report and was looking into it.
Syria's foreign ministry urged the United Nations "to stop these crimes.and end the aggressive, illegitimate presence of American and other foreign forces".
The final push to expel hundreds of diehard jihadists from that patch on the Iraq border was announced on Saturday by the Kurdish-led Syrian Democratic Forces (SDF).
As the fighting against the Islamic State's last enclave in Syria continues, forces there are saying their biggest challenge is trapped civilians.
Trump shocked Washington's allies in December by announcing a pullout of all 2,000 USA troops from Syria.
A USA official said Guantanamo is the "option of last resort".