The killings, for which there were no immediate claim of responsibility, occurred in a town called Muzayrib in Daraa province, the Britain-based Syrian Observatory for Human Rights said.
"Unknown assailants attacked the municipality building... abducting nine members of the security forces before shooting them dead and abandoning their bodies in a square," Observatory head Rami Abdel Rahman said.
Attacks are common in the province, which was retaken by regime forces from rebels in 2018, usually targeting loyalists and civilians working for the state, according to the Observatory.
But the nature of Monday's incident and high toll are unusual.
"Usually attacks against regime forces target checkpoints or patrols, not government buildings," Abdel Rahman said.
Daraa is considered to be the birthplace of the popular uprising that erupted across Syria in 2011, before spiralling into a full-blown conflict.
After its recapture by Russia-backed regime fighters in 2018, state institutions returned but the army is still not deployed in the whole province, says the Observatory, which relies on a network of sources in Syria.
Many former rebels stayed instead of evacuating under a Moscow-brokered deal, either joining the army or remaining in control of parts of the province and some neighbourhoods of the provincial capital, also called Daraa.
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The Islamic State group has in the past claimed attacks in the area, in July saying it had killed six soldiers at a checkpoint.
The jihadists lost the last scrap of their territorial "caliphate" a year ago, but retain a presence in Syria's vast Badia desert.
The civil war has killed more than 380,000 people and displaced millions from their homes since starting with the brutal repression of government protests.
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