The Syrian army and a rebel group in the southern province of Deraa have accused each other of breaching a ceasefire deal agreed two days ago.
Opposition activists said on Sunday that Syrian government air raids targeted a group of rebel fighters in Um al-Mayadin village, just 5km north of the Jordanian border, killing at least four people.
The ceasefire deal - brokered by Russia, a Syrian government ally, on Friday - saw rebels agree to hand over heavy weapons in exchange for security guarantees and safe passage to other areas.
Rebels in Um al-Mayadin refused to give up their arms, a source in Deraa province told Al Jazeera. They accused Moscow of not abiding by the agreement. It was not clear which part of the deal they referred to.
Under the agreement, Russian military police were to be deployed in the Deraa areas previously held by rebels.
It was also agreed that Syrian government forces were to withdraw from four villages in eastern Deraa: Kahil, al-Sahwa, al-Jiza and al-Misaifra. As of Sunday, the Syrian forces had not pulled out of the villages.
Meanwhile, in the western suburbs of Deraa province, clashes between the Syrian army and rebels opposed to the truce deal intensified on Sunday, according to the source inside Syria.
Rebels in Deraa's western areas have formed what they call "the Southern Army", a group of 11 rebel factions, according to a statement they issued on social media on Sunday.
"We call on all factions in the south to join our army in defending our land and dignity," it said in a statement.