Syrian forces launched an attack on rebels in the southern province of Sweida on Monday despite a ceasefire, Syrian Observatory for Human Rights monitor said.
State media reported that the assault was against the militant Islamic State (IS) group.
The Syrian Observatory for Human Rights, a Britain-based monitor, said the assault began on Monday morning, almost 24 hours into an internationally brokered ceasefire.
“The regime started an attack on the area northeast of Sweida city, backed by air strikes,” the monitor said, reporting clashes between government troops and rebels on the ground.
Observatory director Rami Abdel Rahman said the groups under attack have received support from the US-led coalition fighting IS.
He said the regime had captured a string of hilltops and villages in the area.
However, Syrian state media named those same positions as territory that government forces had captured from IS.
“Our forces and allies captured several areas, villages, hilltops and commanding positions in the eastern countryside of Sweida after eliminating large numbers of IS terrorists,” state news agency SANA said.
The clashes come as a ceasefire for southern Syria negotiated by the United States, Russia and Jordan entered its second day.
So far, the deal announced Friday has brought quiet to most of the three provinces covered: Daraa, Quneitra and Sweida.
But in addition to the Sweida clashes, there have been other limited violations according to the Observatory, including regime shelling and exchanges of fire with rebels in Daraa.
In Quneitra province, there were also reports of sporadic machine gun fire from both sides, though there were no casualties in any of the incidents, the monitor said.
Sunday's ceasefire went into effect just ahead of new peace talks in Geneva.
More than 320,000 people have been killed in Syria since the conflict began in March 2011 with anti-government protests.