Islamic State (ISIS) group fighters fired missiles containing toxic gas at Kurdish forces in Syria last month, the forces and a monitoring group said Saturday, according to AFP.
The Kurdish People's Protection Units (YPG) and the Syrian Observatory for Human Rights both reported the late June attacks in Hasakeh province in northeastern Syria, the news agency reported.
The YPG said the attacks occurred on June 28 and targeted the Kurdish-held Salhiya district of Hasakeh city and Kurdish positions south of the town of Tel Brak.
"Upon impact, the projectiles released a yellow gas with a strong smell of rotten onions," a statement quoted by AFP said.
It added that the ground around the impact sites was stained with a liquid that was green at first but turned yellow on contact with sunlight.
"Our troops exposed to the gas experienced burning of the throat, eyes and nose, combined with severe headaches, muscle pain and impaired concentration and mobility. Prolonged exposure to the chemicals also caused vomiting."
The YPG reported no deaths in the attacks and said that exposed forces subsequently recovered from their symptoms.
They added that YPG fighters had captured industrial-grade gas masks from ISIS forces in recent weeks "confirming that they are prepared and equipped for chemical warfare along this sector of the front."
The Observatory also reported the two attacks.
Citing medical sources, the Britain-based monitoring group said at least 12 YPG fighters suffered symptoms including choking, burning eyes and vomiting in the attack south of Tel Brak.
Neither the Kurds nor the Observatory were able to confirm what type of chemical had been used in the missiles, although the YPG said it was carrying out an investigation in conjunction with the Conflict Armament Research group and Sahan Research experts.
On Saturday, the New York Times said the two research groups were also investigating the use of chemical weapons by IS against Kurdish forces in Iraq.
The newspaper said the researchers had found one mortar round containing a chemical that struck a Kurdish military position near Mosul Dam on June 21 or 22.
The chemical was still being analyzed, but one expert told the newspaper he was "certain it is chlorine."
ISIS has been accused of using chlorine against Kurdish forces before.
In March, the autonomous Kurdish government in northern Iraq said it had evidence that the jihadist group used chlorine in a car bomb attack on January 23.
Chemical weapons have been deployed in the Syrian conflict on multiple occasions and not just by ISIS, as the Bashar Al-Assad regime has been accused of using them as well.
By far the deadliest incident, in August 2013, involved the use of sarin gas and killed up to 1,400 in a rebel-held Damascus suburb.
The Syrian opposition and much of the international community blamed the attack on the Damascus government, which denied responsibility.
Following the sarin attack, the United States threatened military intervention against Syria's government, but the Damascus government forestalled foreign intervention by agreeing to a U.S.- and Russian-brokered deal under which it joined the Organization for the Prevention of Chemical Weapons (OPCW), admitting to having a chemicals weapons program and promising to eliminate it.
A joint mission between the UN and the OPCW was then tasked with eliminating Syria's chemical weapons program, and the government of President Bashar Al-Assad last year handed over 1,300 tons of chemical arms.
In May, however, diplomats said that international inspectors had found traces of sarin and VX nerve agent at a military research site in Syria that had not been declared to the OPCW, despite Syria’s pledge to do so.
And last week, the United States asked the UN Security Council to set up an investigative panel to identify those behind deadly chlorine gas attacks in Syria.
The OPCW released a report in January in which it concluded "with a high degree of confidence" that chlorine gas had been used in attacks on three villages in Syria last year, but the organization did not attribute blame for the attacks, as its mandate does not allow for such determinations.