"There have been several allegations of chlorine attacks, in Ghouta, in Idlib, and also now recently in Afrin. While we cannot independently verify these allegations but if confirmed, it is outrageous and should be having no impunity," UN Envoy Staffan de Mistura said on Wednesday.
Turkish-aligned media claimed attacks were carried out by Kurdish People's Protection Units (YPG) in Afrin early this month.
“FSA elements were targeted by the terrorist PYD militia with a shell containing poisonous chlorine gas on the Sheikaruze front north of Afrin,” read an Operation Olive Branch tweet on February 6 with four photos of men claimed to be suffering the after effects of a gas attack.
The YPG issued a strongly worded denial condemning the use of these and similar weapons by the "occupying Turkish state and their terrorists."
"On the contrary, the Turkish army, which suffered heavy blows during our operations in the Sheikaruze area, bombed the area with artillery shells from chemical weapons," stated the YPG in a response.
Turkey's military, including its air forces and backed by Free Syrian Army (FSA) proxy fighters, began its assault on the Syrian governorate of Afrin on January 20.
As of Wednesday, the UK-based Syrian Observatory for Human Rights documented the deaths of 77 civilians. Local officials have reported higher civilian casualty figures.
"Our forces moved to high points after being subjected to the effects of the weapon during conflict, and the Turkish-backed terrorists left on the ground were affected by the weapon themselves," added the YPG at the time.
The Turkish operation has controlled about 7 percent of the canton, according to the Observatory.
Through the seven-year Syrian civil war there have been numerous reports of chemical weapons attacks in Syria, but the UN-backed fact-finding teams have complained of a lack of access to the sites.
In April 2017, US President Donald Trump unilaterally ordered a cruise missile attack against a Syrian air base in Idlib. That followed an alleged sarin gas attack on the regime-opposition town of Khan Sheikhoun and killed at least 90 people.
Due to the lack of progress through the UN, 24 countries endorsed the International Partnership against Impunity for the Use of Chemical Weapons in Paris in late January.
"The current situation cannot continue," France's Foreign Minister Jean-Yves Le Drian said at the time.
The partnership will share information on the topic and create dossiers of those believed to be involved in using chemical weapons.
Syria and its primary backers, Russia and Iran, have denied using internationally banned weapons in the seven-year conflict.
"Syria's government categorically denies possessing ... chemical weapons," Deputy Foreign Minister Faisal Mekdad said on Wednesday. "We consider the use of such arms as immoral and unacceptable, whatever the context," he added.
He was responding to threats on Tuesday by France's President Emmanuel Macron, who said: "On chemical weapons, I set a red line and I reaffirm that red line."
"If we have proven evidence that chemical weapons proscribed in treaties are used, we will strike the place where they are made," he added.
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