Syrian rebels held onto a commanding position in a mountain range in the coastal province of Latakia, the ancestral home of Syrian President Bashar Assad, after government forces were forced to withdraw.
They said the army's attempt was the latest of several costly campaigns to seize Kubayna, after it mounted an offensive last month with Russian air power to retake main highways and trade arteries around Idlib and northern Hama now in rebel hands that have fragmented the country's war-torn economy.
"Whoever controls Kubayna ensures a large stretch of territory is effectively under their firing range. The regime wants it to protect its coastal villages from rebel fire," said Major Youssef Hamoud, spokesman for the Turkey-backed group of mainstream rebels called the National Army.
An official from Tahrir al-Sham, the latest incarnation of the former Nusra Front which was part of Al-Qaida, said poison gas was used in the army's attack on their position on the mountain slopes in an attempt to regain control.
Abu Baraa al-Shami, a fighter based there, told Reuters that several fighters suffered choking symptoms.
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The army denied the claim and said it was continuing to fight terrorism, with state media earlier saying the military had struck at Al-Qaida terrorists in the last jihadist foothold in Latakia province that has long been a launching pad for drone attacks on the main Russian base of Hmeimim nearby.
The eviction of jihadists from commanding positions in the mountains would bring the army closer to securing parts of Idlib and a main highway that connects the cities of Latakia and Aleppo.
The fighting has continued even after Russia agreed with Turkey to a 72 hour halt following an upsurge in violence in northwest Syria that has sparked an exodus of tens of thousands to the safety of border areas with Turkey, residents and opposition sources have said.
Russia's defense ministry confirmed on Sunday a "unilateral cease-fire" in the Idlib buffer zone in a move the opposition said showed the failure by Moscow and the army after almost three weeks \intensive strikes to bring a rapid collapse in rebel lines.
"They are facing stiff resistance in areas that had fallen to the army," said Hamoud, adding many of their fighters from a nearby stretch of territory to the north protected by the Turkish army had joined their compatriots in fronts.
The army has so far gained three significant areas, the last being the town of Hawayz on Friday after taking Qalaat al Madiq and the town of Kfar Naboudah.
Two senior Western diplomats following Syria say the aim appears to be to take control of the main cities of Maarat al-Numan and Khan Sheikhoun on the main highways in Idlib.
The campaign that began in earnest late last month has also killed dozens, destroyed hundreds of civilian homes, more than a dozen hospitals and food stores, according to opposition-based rescuers and Western aid agencies.
Both Moscow and Damascus deny indiscriminate bombing of civilians and say they seek to crush radical Islamist groups.