According to reports, the blast occurred on Friday on the southern edge of the rebel-held city of Idlib with seven members belonging to the al-Qaeda-linked Hayat Tahrir al-Sham.
They agreed on further steps to respect the agreement on creating such a demilitarized strip, but at the same time measures to prevent extremists from attempts to sabotage this important agreement
The deadly jihadist assault came hours after government troops killed 23 fighters of a formerly US-backed rebel group inside the planned buffer zone.
The accord also provides for the clearance of heavy weapons from a U-shaped zone around the northwestern region of Idlib, but a monitor said rebels late Saturday fired mortar shells from it.
The Britain-based war monitor did not immediately report any casualties from the bombardment which came after shelling and air strikes killed at least 15 civilians in the rebel zone since September 4.
Since 2015, Idlib has been home to a complex array of anti-regime forces: secular rebels, Islamists, Syrian jihadists with ties to Al-Qaeda -- and their foreign counterparts.
Jeffrey said an attack by Russian and Syrian forces, and the use of chemical weapons, would force huge refugee flows into southeastern Turkey or areas in Syria under Turkish control.
Troops have been massing on the edges of the northwestern province on the border with Turkey for weeks, raising fears of a humanitarian disaster on a scale not yet seen in Syria's seven-year conflict.
Donald Trump on Monday sent a tweet warning the Syrian government and its allies against a "reckless attack" on rebel-held Idlib province.
Regime ally Moscow and rebel backer Ankara have held several rounds of talks aimed at averting an assault, but government troops have been massing near the rebel zone.
Earlier Monday, Iran called for militants to be "cleaned out" of Idlib, as it prepared for talks with Syria and Russia about confronting the last major enclave held by rebels opposed to Assad.
The pontiff said there were still "winds of war" and "unsettling news about the risk of a possible humanitarian catastrophe in the province of Idlib."
In recent days, both the government and its ally Russia have stepped up their rhetoric against the rebel presence in Idlib, which is dominated by the Hayat Tahrir al-Sham
Since Syria's conflict erupted in 2011, more than 350,000 people have been killed, more than 11 million have fled their homes and medical infrastructure has been systematically targeted.
The draft resolution "condemns in the strongest terms the use of chemical weapons" in Syria.