Syrian rebels held onto a commanding position in a mountain range in the coastal province of Latakia, the ancestral home of Syrian President Bashar al-Assad, after government forces were forced to withdraw, Reuters reported.
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.
The northwest represents the last big piece of territory held by rebels opposed to Assad. The coastal province of Latakia is home to the Assad family's Alawite minority.
"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 Qaeda, 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.
The army denied the claim and said it was continuing to fight terrorism, with state media earlier saying the military had struck at al Qaeda 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.