Syrian government forces launched an attack on rebels in a southern province on Monday despite a ceasefire, but state media said the assault was against the Islamic State group.
The Syrian Observatory for Human Rights, a Britain-based monitor, said the assault in Sweida province began in the 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 rebel fighters on the ground.
One of the targeted groups, the Ahmad al-Abdo Army, said "fierce fighting" was ongoing.
Regime forces "launched a big attack today, with huge columns of armoured vehicles and with Syrian air cover," said Firas al-Munjed, the group's communication head.
Both he and the Observatory said regime forces had captured a string of hilltops and villages along the border between the provinces of Sweida and Rural Damascus.
But 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," said the official SANA news agency.
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 machinegun 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.