BEIRUT (AP) — Syrian government airstrikes and shelling in the country's south killed at least 18 people on Thursday, including three children, opposition activists said.

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

Thursday's shelling struck an area known as Daraa al-Balad in the city of Daraa. The Britain-based Syrian Observatory for Human Rights said 18 people were killed while an activist in Daraa province, Ahmad Masalma, put the number at 21. The activists said the death toll is likely to rise because many of the wounded were in critical condition.

Busra 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 four-year-old conflict. It was considered to be a stronghold of pro-government forces in Daraa province.

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

Heavy fighting raged around Idlib in northwestern Syria on 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. It said at least 11 government fighters and 17 rebels have been killed over the past 24 hours.

The offensive is being led by several groups, including al-Qaida's branch in Syria, known as the Nusra Front, as well as the ultra-conservative 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.

Late Wednesday, Syria's state news agency said the army killed "tens of terrorists" on the eastern side of Idlib. The government refers to Assad's opponents as "terrorists."