A UN commission on Wednesday called on rebel groups in Syria's Idlib province to leave urban areas to protect civilians from any looming regime assault.
The proposal comes after the United Nations' peace envoy for Syria, Staffan de Mistura, last week suggested a deadline be set for fighters in Idlib to pull back from its cities.
UN agencies and relief organisations have warned repeatedly that any major assault on the province of Idlib, which borders Turkey, could spark one of the worst humanitarian disasters of Syria's seven-year war.
On Wednesday, the UN Commission of Inquiry on Syria called for jihadists and opposition fighters to leave the most densely populated areas in the region where some three million people live.
"Most of those terrorist groups and other armed groups, they are in the cities. Perhaps one wonderful scenario is: leave the cities," commission chief Paulo Pinheiro said.
Hany Magally, a fellow panel member, said: "Shouldn't the armed groups move out and spare the civilian population?"
Idlib and adjacent areas are largely controlled by Hayat Tahrir al-Sham, an alliance led by Al-Qaeda's former Syrian affiliate, as well as rival rebels. HTS controls the provincial capital Idlib city.
The northwestern region has seen its population almost double with the arrival of Syrians displaced from other parts of the country, many of whom already depend on aid.
"All the other disasters would be minor events compared to what can happen in Idlib," Pinheiro said.
- 'We've come back home' -
On Wednesday, intermittent artillery fire hit southern districts of Idlib province and adjacent rebel-held areas of Hama province, a Britain-based monitor said.
But for the second day in a row there were no air raids, after deadly strikes and barrel bombing at the weekend, said the Syrian Observatory for Human Rights.
In the south of Idlib, a man told AFP he and his family had returned to their home after fleeing for their lives on Monday.
"I fled two days ago after the regime targeted us with air strikes and barrel bombs," Abu Ammar said in the southern district of Hobait.
"But with warplanes on hold since yesterday, we've come back home."
Since September 4, shelling and air strikes by the regime and Russia have killed at least 15 civilians in the rebel-held zone, the Observatory says.
On Friday, rebel backer Turkey and regime allies Russian and Iran failed to reach an agreement in Tehran to avoid a regime assault on Idlib.
On Tuesday, UN chief Antonio Guterres warned a full-scale battle on Idlib "would unleash a humanitarian nightmare unlike any seen in the blood-soaked Syrian conflict".
More than 350,000 people have been killed and millions more displaced in Syria's war since it started in 2011.
- IS hostages -
In the eastern province of Deir Ezzor, US-backed fighters were battling to oust IS from the town of Hajin on the east bank of the Euphrates, the most significant remnant of the jihadists' "caliphate" which once spanned Syria and Iraq.
The operation "will clear remnants of (IS) from northeastern Syria along the Middle Euphrates River Valley toward the Syria-Iraq border," the US-led coalition said.
Since Monday, the battle for Hajin has killed 36 jihadists and 11 fighters from the US-backed Syrian Democratic Forces, the Observatory said.
Despite a number of military campaigns against them, IS fighters are still present in Deir Ezzor, as well as in the vast desert that stretches from Damascus to the Iraqi border.
Regime forces have been battling IS for weeks in the southern province of Sweida, ever since the jihadists killed more than 250 people in attacks on its provincial capital and nearby villages in late July.
During their rampage, IS fighters also kidnapped around 30 people, most of them women and their children from the Druze religious minority.
IS has since announced the death of an elderly woman and the execution of a 19-year-old male student.
According to a local source, families of a number of hostages received a video on Wednesday showing around 30 people.
In the footage, which was seen by AFP but could not be independently verified, a woman says she is speaking on Tuesday and that the group is being held by IS.
She accuses the Damascus regime and its Russian ally of inaction over the kidnapping and appeals for help to secure the group's release.
Late Monday, 21 regime fighters were killed in an IS ambush in Sweida's volcanic plateau of Tulul al-Safa.