BEIRUT/MOSCOW: A roadside bomb and government shelling killed at least 19 people, most of them rebel fighters, in southern Syria’s Deraa province Thursday, an activist group said.

The Britain-based Syrian Observatory for Human Rights said an explosive device placed on a road by government forces detonated as a convoy of rebel fighters passed by.

“After the blast, civilians from nearby came to the scene and the regime shelled the area,” Observatory Director Rami Abdel-Rahman said. He said 13 of the dead were from local rebel factions, and at least three others were civilians.

The Observatory said the toll in the incident in the northwest of the province could rise further because several people were suffering serious injuries.

Deraa province is one of the last remaining bastions of rebel forces, who retain control of a majority of the region even as they have been routed elsewhere.

In other development, Russia said Thursday it was systematically bombing any Daesh (ISIS) militants trying to flee Raqqa and had carried out two such bombing runs in the last week.

A U.S.-led coalition is supporting an alliance of Kurdish and Arab rebels in their campaign to capture Raqqa city, Daesh’s urban base of operations in Syria. But the Russian Defense Ministry said Thursday that the Syrian Democratic Forces, which includes the powerful Kurdish YPG militia, had halted their offensive, leaving gaps on Raqqa’s southern edge that Daesh militants were using to try to leave the city and regroup further south. It said Russian planes had destroyed a militant convoy headed for Palmyra from Raqqa on May 25, and had bombed three other convoys late on May 29 and in the early hours of May 30.“Islamic State’s losses included more than 80 terrorists, 36 vehicles, eight fuel tankers, and 17 pickup trucks mounted with mortars and machine guns,” the ministry said in a statement, using another name for Daesh.

“Any attempts by Islamic State militants to leave Raqqa in the direction of Palmyra will be harshly stopped,” it said.

Meanwhile, about 10,000 civilians have fled to a camp just north of Raqqa with hundreds more arriving each day as the battle for the city nears, Doctors Without Borders (MSF) said Thursday. Residents are escaping Raqqa under cover of night as U.S.-backed forces close in, taking their chances against minefields and hostile fighters rather than risking death in a major battle expected to begin soon.

“It is not a massive exodus, but about 800 people ... are arriving in Ain Issa every day,” Natalie Roberts, an emergency doctor from MSF France who had just returned from the region, told reporters. The camp in Ain Issa village is run by the SDF who have now arrived to within 3 kilometers of Raqqa and plan to press on with the assault on Daesh. The SDF had planned to make the camp a transit point for civilians. But the need to register each person and with many not having an alternative destination to travel to, the camp has expanded to beyond its 6,000-person capacity, Roberts said. Conditions have deteriorated especially due to the summer heat. The U.N. and other aid groups have yet to establish themselves in the zone, she said.

MSF is providing basic care, while people with war wounds, mostly caused by mines, are being sent to three MSF hospitals further north.

It is unclear how many people remain in Raqqa. The city’s population was estimated at 200,000 before the recent departures.