Rebel groups fighting under the Free Syrian Army banner accuse the Jabhat Fateh al-Sham group of launching a surprise attack on their positions west of Aleppo.
Russia and regional powers Turkey and Iran backed a shaky truce between Syria's warring parties on Tuesday and agreed to monitor its compliance, but on the ground rebels faced continued fighting on two fronts which could undermine the deal.
After two days of deliberations in Astana, Kazakhstan's Foreign Minister Kairat Abdrakhmanov said the powers had agreed in a final communique to establish a system "to observe and ensure full compliance with the ceasefire, prevent any provocations and determine all modalities of the ceasefire."
While welcoming the text, the Syrian regime's chief negotiator Bashar Jaafari said an offensive against rebels west of Damascus would carry on. Rebels say it is a major violation of the ceasefire that took effect on December 30.
Opposition negotiator Mohammad Alloush in turn said he had reservations about the text which according to him legitimised Iran's "bloodletting" in Syria and did not address the role of Iran-backed militias fighting rebels.
The three foreign powers also said they supported the willingness of the armed opposition groups to participate in the next round of negotiations to be held in Geneva on February 8.
TRT World's Hasan Abdullah adds more from Astana.
Syrian rebels battle Jabhat Fateh al-Sham in northern Syria
In northwest Syria, heavy fighting erupted between the Jabhat Fateh al-Sham group and Free Syrian Army (FSA) factions who were represented at the Astana talks. The clashes threaten to further weaken the opposition to Syrian regime leader Bashar al-Assad in its biggest territorial stronghold.
Rebel groups fighting under the FSA banner accused Jabhat Fateh al-Sham of launching a surprise attack on their positions west of Aleppo.
Fateh al-Sham could not immediately be reached for comment. The group was previously known as the Nusra Front, a name it changed after announcing it was cutting ties with Al Qaeda last year.
FSA officials said the attack targeted FSA groups in areas west of Aleppo and the adjacent province of Idlib, which is controlled by an array of rebel factions, including Fateh al-Sham.
One FSA commander said that in response "a comprehensive war" was now underway against Fateh al-Sham.
Long-standing tensions between Fateh al-Sham and more moderate rebels, a number of them backed by Turkey and other states that have opposed Assad, have surfaced again since regime forces helped by Russian air power and Iranian-backed militias drove the rebels out of Aleppo last month.
Fateh al-Sham is not covered by the truce between the regime and rebels brokered by Russia and Turkey. The aim of the meeting in Astana, organised by Russia, Turkey and Iran, is to shore up the ceasefire that came into effect on December 30.