Shells fired by Syrian opposition factions struck Sweida city in the South on Tuesday for the first time since 2015, said the Syrian Observatory for Human Rights.
On Tuesday, the monitor said opposition forces fired shells into Sweida city, "which led to loud blasts but no casualties".
"It is the first time since 2015 that the city has been subjected to shellfire," said Observatory chief Rami Abdel Rahman, according to AFP.
The regime holds most of Sweida province, but opposition factions still control much of the nearby Daraa and Quneitra governorates.
Sweida, whose residents are mostly from the Druze minority, has remained relatively insulated from seven years of war that ravaged the rest of the country.
But opposition groups hold a sliver of territory in western Sweida that borders their main bastion in the province of Daraa, and clashes and exchanges of fire have erupted in that area in recent days.
Syria's regime has set its sights on ousting rebels from the south and has been dispatching troops and equipment there for weeks.
Opposition faction commander Abu Hassan told AFP on Tuesday his units had seen the reinforcements and were on high alert.
"We are almost always mobilized. The joint operations room has upped its coordination to the highest level," he said.
On Tuesday, the regime dropped new flyers on the rebel-held half of Daraa city, calling on residents to expel the opposition, "like your brothers did in Eastern Ghouta and Qalamun," referring to two areas near Damascus recently recaptured from the opposition.
Opposition fighters appeared to fear the regime would use Sweida's civilian population as justification for the assault, and issued a message addressed to them on Tuesday.
"We call on our people in Sweida province not to serve as bait for the goals of the regime, sectarian militias from Iran, and ‘Hezbollah’, which are trying to occupy this land and divide its people," they said in a statement.
