BEIRUT: Syrian government airstrikes and shelling in the country’s south killed at least 22 people Thursday, including three children, opposition activists and a monitoring group said.

The attacks come amid heightened clashes in southern Syria between government forces and rebels who seized the nearby historic site of Busra al-Sham the previous day.

A barrel bomb struck the area of Deraa al-Balad in the city of Deraa. The Britain-based Syrian Observatory for Human Rights said 22 people were killed, adding that the death toll was likely to rise as many of the wounded were in critical condition.

Busra al-Sham, a town classified as a UNESCO World Heritage Site for its historic citadel and well-preserved Roman theater, had been in the hands of President Bashar Assad’s troops throughout the conflict. It was considered to be a stronghold of pro-government forces in Deraa province.

Rebels have posted photographs and video footage of militiamen entering the theater and inspecting it for damage.

The fighting in southern Syria coincided with rebel gains in the north near the city of Idlib, where an alliance of rebels launched an offensive earlier this week.

Heavy fighting raged around Idlib in northwestern Syria Thursday as rebels pressed ahead with their offensive against the government stronghold. The opposition groups have advanced through the outskirts of Idlib, the provincial capital and home to some 165,000 people, since launching their campaign Tuesday. Rebels have long controlled most of the surrounding towns and countryside, but Assad’s forces have maintained their hold on the city.

The Observatory said the rebels have seized at least 17 checkpoints and military outposts from pro-Assad troops in clashes that have cost at least 71 lives.

Citing eyewitnesses, the Observatory said heavy shelling by the Islamist fighters had forced “regime forces to erect new checkpoints and barricades” in the city.

“The violent clashes are ongoing for a third continuous day, and the regime and [pro-government militia] National Defense Forces are trying to regain the initiative despite the severity of the offensive,” the Observatory said.

It would be only the second provincial capital lost by the regime after Raqqa, which is now the de facto capital of ISIS.

Since Tuesday, the clashes have left at least 37 rebels and 34 fighters from pro-regime forces dead, the Britain-based Observatory said.

Among those killed was Abu Jamil al-Qutb, the second-in-command of the Ahrar al-Sham militia.

The offensive is being led by several groups, including Al-Qaeda’s branch in Syria, the Nusra Front, as well as the ultraconservative Ahrar al-Sham and Jund al-Aqsa groups.

A photograph posted Thursday on a Twitter account run by the Nusra Front’s Idlib unit showed what it said was the group’s black flag being raised over a captured government checkpoint north of the city.

Government aircraft also pounded several areas of rural Idlib province, including the village of Sarmeen, where at least three people were killed, according to the Observatory. Sarmeen has been the site of several recent chlorine gas attacks by regime forces, activists say.

Fierce clashes between regime troops and their allies and rebel groups were reported in the city of Aleppo and in a strategic area north of the city, while regime aircraft pounded the eastern city of Deir al-Zor with at least 14 airstrikes targeting ISIS. In recent days ISIS militants have stepped their attacks on the military airport on the city’s outskirts.

Also, a car bomb targeted the city of Hassakeh killing at least one Kurdish policeman, the Observatory said. The city was rocked by a car bomb last week that killed some 50 people in an attack claimed by ISIS.