Selim Tosun, from the IHH Humanitarian Relief Foundation, warned that if regime forces seized Idlib, there would be many people already displaced who would have nowhere to go.
The state-affiliated Al-Ikhbariya TV says government forces took Sinjar on Sunday.
At least 18 people have been killed in a auto bomb blast in Idlib, Syria's largest rebel-held city, activists and a war monitoring group said.
Sam Heller, a Beirut-based fellow with The Century Foundation think tank, said the regime's corralling of opposition refugees and fighters into Idlib was primarily a strategy to demonstrate that the alternative to it was hardline jihadist rule.
The monitor said there were conflicting accounts, attributing the explosion that shook the Thalatheen district of the city to either a vehicle bomb or a drone attack.
The Syria Civil Defence, whose rescue workers are known as the White Helmets, says four children and 11 women were among the 25 people killed.
The nature of the attack was not immediately clear. They need to keep USA support, including several bases in Kurdish-held territory, as a guarantee against Turkish military intervention or an offensive by Syrian forces.
Tahrir al-Sham Front held an emergency meeting of its military council, chaired by its leader Abu Mohammad al-Golani, according to the group's official Telegram channel.
Assad's forces have captured about 60 towns and villages on the border between Idlib and neighboring Hama province since the start of the operation in October 2017.
Since then, hundreds of thousands of people have been killed and more than 10 million have been forced from their homes, according to the United Nations.
The sources also reported that heavy fighting has erupted between the civilians and Al-Nusra terrorists in Asqat region in the town of Saqlin in Northern Idlib.