Unidentified gunmen killed a senior Nusra Front commander on Monday in the southern city of Daraa, activists reported.
Nusra Front's security official in Daraa, Ahmed Mohamed al-Falouji, was assassinated after an unknown group broke into a field hospital where he was receiving a treatment, shooting him dead.
In Daraa, Saber Safar, a Free Syrian Army commander, narrowly escaped an assassination attempt on Saturday.
The key rebel group Ahrar al-Sham Movement has also been under fire amid a mounting number of senior fighters who were killed by unidentified groups.
Activists and rebels groups say ISIS and the Syrian regime is behind the recent spate of murders and attempted killings.