Twenty-two Syrian regime fighters were killed on Friday in clashes with extremists near a planned buffer zone in the northwestern Idlib province, said the Syrian Observatory for Human Rights.
Early Friday, radical groups attacked regime forces in the northwest of Hama province near the planned zone, it added.
The attackers included the Al-Qaeda-linked Hurras al-Deen group, which has publicly rejected the Russian-Turkish deal, said Observatory chief Rami Abdul Rahman.
It was originally reported the eight fighters were killed, but it increased after the bodies of the missing were found.
The monitor said it was the highest regime death toll from the sporadic clashes around the proposed zone since the deal was agreed.
Turkey and Russia agreed to set up demilitarized zone around Idlib to protect it from a regime assault. Its implementation has been stalled since extremist groups, who hold around 70 percent of the planned buffer zone failed to withdraw by mid-October, and intermittent clashes have since rocked the area.
The lion's share of Idlib is held by Hayat Tahrir al-Sham, an alliance led by Al-Qaeda's former Syrian affiliate.
State news agency SANA reported a military source as saying that "a terrorist group tried to infiltrate one of our advanced position near Sarmaniya" in Hama, killing and wounding regime fighters, without giving a death toll.
Under the September 17 deal, all fighters in the zone were supposed to withdraw their heavy weapons and extremists, including HTS and Hurras al-Deen, were supposed to leave.
On Thursday, Russian spokeswoman Maria Zakharova criticized "sporadic clashes", as well as "provocations" by HTS in northwestern Syria.
Syria's war has killed more than 360,000 people since it erupted in 2011 with the brutal repression of anti-regime protests.
