BEIRUT (AP) — An explosion shook the base of an al-Qaida-linked group in northern Syria on Tuesday, killing at least eight gunmen and wounding others, opposition activists said.

The explosion came as opposition fighters suffered a major setback in southern Daraa province, where a Russian-brokered deal led to the evacuation of gunmen from the region. The evacuation was to pave the way for the deployment of government troops.

It wasn’t immediately clear what caused the explosion at the base of Hayat Tahrir al-Sham in the northwestern province of Idlib, the last major rebel stronghold in the country, but some said it was a shell that exploded as fighters trained. HTS is the most powerful group in Syria’s northwest.

The Britain-based Syrian Observatory for Human Rights said the explosion near the village of Ram Hamadan was apparently caused by a shell that exploded. The Observatory said eight fighters were killed and 10 were wounded.

It added that the explosion occurred as drones of the U.S.-led coalition were flying overhead.

Step news agency, an activist collective, said at least nine fighters were killed and others were wounded in the blast. Step said it could have been caused when a mortar shell exploded during training.

Hayat Tahrir al-Sham, Arabic for Levant Liberation Committee, has militarily crushed many of its rivals within the opposition in recent years.

Nearly a dozen opposition fighters left Daraa province, which has been witnessing clashes for weeks. They were some of the most serious challenges to a tenuous 2018 deal between the government and insurgents negotiated by Russia. Daraa is where the country’s conflict began 10 years ago.

Russia mediated a new deal to end the recent fighting in Daraa, in which insurgents who reject the Russia-brokered deal will have to leave the region.

On Tuesday night, the first batch of fighters consisting of 10 gunmen left the area and headed to rebel-held parts in the country’s north, according to the Observatory.

Pro-government news outlets reported that some opposition fighters had left and that shortly afterward other gunmen opened fire leaving several people wounded. The Observatory confirmed that shooting occurred but blamed government forces.

After dozens more opposition fighters leave the city of Daraa in the coming days, government forces are expected to enter the old part of the city known as Daraa al-Balad. It has for years been a rebel stronghold and has been under tight government siege for weeks.

Opposition fighters who accept the deal will have to hand over their weapons in return for amnesty.

On Tuesday, U.N. Special Envoy for Syria Geir Pedersen in a briefing to the Security Council, called on all parties to end the violence immediately and to meet their obligations under international humanitarian law to protect civilians and civilian infrastructure.

“Immediate, safe and unimpeded humanitarian access is needed to all affected areas and communities, including Daraa al-Balad,” Pedersen said.

Daraa province became known as the cradle of the uprising against Syrian President Bashar Assad that erupted in 2011 as part of the Arab Spring revolts. It was recaptured by Syrian government troops in 2018. Assad has since regained control of most of the country with the help of Russia and Iran.

The Russian-mediated deal in 2018 allowed some of the province’s armed opposition to remain in their former strongholds, in charge of security. Government troops retained control of the province, but security duties were divided. Tensions regularly erupted and government troops tried several times to take over areas under opposition control.

Syria’s 10-year conflict has left about half a million people dead and half the country’s prewar population of 23 million displaced, more than 5 million of them as refugees outside the country.