A rival Syrian insurgent commander has reportedly accused the Islamic State (ISIS/ISIL) of using “toxic gases” while shelling the rebel-held town of Marea in northern Aleppo province, about 12 miles from the Turkish border.

ISIS was also accused of using chemical agents, most likely mustard gas, against over 50 civilians in Marea last Friday, according to the medical charity Medecins Sans Frontieres and the Syrian American Medical Association.

Moreover, rebel and Islamist factions combating ISIS in Aleppo accused the jihadist group on Thursday of using toxic gases while shelling their positions in the village of Harjalah on the Turkish border.

The Guardian recently quoted unnamed Pentagon officials as saying that they had seen no “confirmable links” indicating that ISIS had used chemical weapons in Aleppo, without specifically ruling out the attack.

Evidence unveiled by doctors and experts suggesting that ISIS used chemical agents against civilians in Marea “raises the prospect, denied by the Pentagon, that ISIS has gained access to chemical stockpiles from Syrian President Bashar al-Assad’s regime thought to have been destroyed or degraded” under a 2013 deal brokered by Russia, noted The Guardian.

Rebel-held territory along the Turkish border in Syria has reportedly fallen to the Islamic State.

Seizing the northern Syria land has allowed ISIS to advance in an area where Turkey and the United States are planning to launch a new offensive against the jihadist group in coordination with insurgents on the ground.

Jordan recently indicated that it is willing to join the U.S.-Turkey offensive.

The UK-based Syrian Observatory for Human Rights (SOHR), a monitor group that uses sources on the ground to track the Syrian conflict, suggested that there are Islamist elements among the insurgents on the ground, who may be asked to join the Turkey-U.S. offensive against ISIS.

According to the Observatory, ISIS has seized five villages, including two near the Turkish border, from the rebels and Islamist factions on the ground in northern Aleppo, described by Reuters as “likely partners for Ankara and Washington in any ground campaign.”

Losing the strategic town of Marea would be a major blow to the insurgents and Islamists fighting ISIS, which include Free Syrian Army (FSA) fighters, NBC News reported.

As of August 27, Marea remained in FSA hands, aded the report.

“If [ISIS] progress continues, the northern countryside of Aleppo could fall,” a rebel told Reuters on condition of anonymity.

“If Marea falls, it means the fall of an important symbol of the groups fighting Daesh,” he added, using an Arabic acronym for ISIS.

On Monday, Turkish Foreign Minister Mevlut Cavusoglu told Reuters that his country and the United States were planning to launch “comprehensive” air operations to expel ISIS jihadists from the border region.

“The two NATO allies plan to provide air cover for what Washington judges to be moderate Syrian rebels, in a joint operation to drive [ISIS] from a rectangle of border territory roughly 80 km (50 miles) long,” noted Reuters. “U.S. jets have already begun air strikes from Turkish bases in advance of the campaign.”

“Diplomats familiar with the plans say cutting [ISIS] access to the Turkish border, across which the radical group has been able to bring foreign fighters and supplies, could be a game-changer,” it added.

Peter Cook, the Pentagon’s press secretary, told reporters earlier this week the United States and Turkey had finalized the details for Turkey’s full participation in the anti-ISIS coalition operations.

“This includes full integration into the coalition’s air tasking order, which tracks, coordinates, and de-conflicts all coalition air operations,” he explained, later adding, “Beyond air operations we continue our dialog with Turkey to evaluate options on the most effective means of countering ISIL, including along its borders in a manner that promotes Turkey’s security and regional stability.”

Cook described Turkey’s participation in the air campaign as a significant step forward.

“That being said, our cooperation with the Turks and–and the expansion of that corporation remains a work in progress at this point,” he added.

Cook said the U.S. and Turkey have not discussed the creation of a safe zone along the Syrian border.