Ceasefire negotiations have started in Syria's southern province of Daraa after the deadliest flareup in three years killed 28 people, including 11 civilians, a war monitor said Friday.

The clashes on Thursday between government forces and rebel fighters marked one of the deadliest days of fighting in Syria in recent months.

They posed a fresh challenge to a Moscow-brokered truce that allowed rebels to stay in Daraa province after it was taken by the government in 2018.

On Friday, the two sides exchanged tit-for-tat machine gunfire but refrained from escalating attacks following talks to stop the fighting, the Syrian Observatory for Human Rights said.

The negotiations between the Syrian army, security officials and a committee of Daraa residents formed in the wake of the clashes, came under pressure from Russia and it remains to be seen whether a ceasefire deal was reached, the monitor said.

Thursday's exchange started when the army launched a ground assault backed by artillery on Daraa al-Balad, a southern district of the provincial capital that is considered a hub for former rebel groups, the Observatory said.

In response, rebel fighters launched a broad counterattack, seizing several government positions across the province and capturing more than 40 troops and militiamen, it added.

The violence killed 28 people, including eight government fighters and nine gunmen affiliated with opposition groups.

Shelling by the army of several parts of Daraa also killed 11 civilians, including several children.

The Observatory said the clashes were the fiercest and most extensive to hit Daraa since it came under governemnt control.

Russian-backed Syrian troops and allied forces recaptured Daraa from rebels in 2018, a symbolic blow to the anti-government uprising born in the city in 2011.

Many former rebels stayed on in Daraa, spurning evacuation under a Moscow-brokered deal. Some joined the army, others have remained in control of parts of the province.

State institutions have since returned but the army has still not deployed province-wide and tit-for-tat bombings and assassinations have become routine.

In March, gunmen in Daraa killed 21 Syrian soldiers in a bus ambush.

The soldiers were en route to Al-Mzairib district in the rural west of the province to arrest a former rebel commander when they came under fire.

The war in Syria has killed nearly 500,000 people and displaced millions since it started in 2011.