A September deal between government ally Russia and opposition backer Turkey aimed to set up a de-militarised zone around the northwestern region of Idlib to protect it from a regime assault.
But its implementation has been stalled since militants who hold around 70 percent of the planned buffer area failed to withdraw by mid-October, and sporadic clashes have rocked the area since.
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Early on Friday, militant groups attacked government forces in the northwest of Hama province near the planned buffer zone, the Britain-based Syrian Observatory for Human Rights said.
"Nine regime fighters and five assailants were killed" in the attack, causing government forces to respond with artillery fire, Observatory head Rami Abdel Rahman said.
The attackers included the Al-Qaeda-linked Hurras al-Deen group, which has publicly rejected the Russian-Turkish deal, he said.
The lion's share of Idlib is held by Hayat Tahrir al-Sham, an alliance led by Al-Qaeda's former Syrian affiliate.
Under the September 17 deal, all fighters in the zone were supposed to withdraw their heavy weapons and militants including HTS and Hurras al-Deen were supposed to leave.
On Thursday, Russian spokesperson Maria Zakharova criticised "sporadic clashes", as well as "provocations" by HTS in northwestern Syria.
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Late last month, Syria's Foreign Minister Walid Muallem expressed dissatisfaction with the implementation of the Idlib deal, and criticised Turkey for shortcomings.
He said heavy weapons had not been withdrawn and accused Turkey of not wanting to "respect its obligations".
Syria's war has killed more than 360,000 people since it erupted in 2011 with the brutal repression of anti-government protests.
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