The Islamic State group fired chemical weapons against Kurdish forces in Syria and Iraq last month, Kurdish fighters and weapons experts have said.
The Conflict Armament Research (CAR) group and Sahan Research said in a statement Friday that IS targeted Iraqi Kurdish peshmerga with a projectile filled with a chemical agent on June 21 or 22.
The organisations also documented two such attacks against Kurdish fighters from the People's Protection Units (YPG) in Syria's northeastern Hasakeh province on June 28.
The YPG said the attacks 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," the YPG said in a statement on Friday.
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 IS forces in recent weeks "confirming that they are prepared and equipped for chemical warfare along this sector of the front."
CAR and Sahan Research, who conducted research in coordination with Kurdish forces, said in a joint statement that seven projectiles were fired in the Hasakeh city attack, and 17 in the attack near Tal Brak.
- 'Projectile-delivered chemical agents' -
They said urine samples taken from those affected in the Tal Brak attack tested positive for a compound that is commonly found in agricultural pesticides.
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But they had no definitive answer yet on the precise chemical composition of the agents that had been used in the two attacks in Syria.
The chemical used in the Iraq attack had characteristics and clinical effects "consistent with a chlorine chemical agent," the groups said.
They said the three attacks in the two countries were "the first documented use by IS forces of projectile-delivered chemical agents against Kurdish forces and civilian targets."
"Although these chemical attacks appear to be test cases, we expect IS construction skills to advance rapidly as they have for other IEDS (improvised explosive devices)," said Emmanuel Deisser, managing director at Sahan Research.
IS has been accused of using chlorine against Kurdish forces in Iraq 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 also been deployed in the Syrian conflict on multiple occasions.
By far the deadliest incident, in August 2013, involved the use of sarin gas and killed up to 1,400 people in a rebel-held Damascus suburb.
The Syrian opposition and much of the international community blamed the attack on the Damascus government.
Syria denied responsibility but subsequently surrendered its arsenal of chemical weapons under a UN-backed agreement.
There have since been a number of reported attacks using chlorine.
Some have been confirmed by the international chemical weapons watchdog but it did not say who carried them out.
Syria was not required to surrender any stocks of chlorine under the 2013 agreement because it is widely used for commercial and domestic purposes.
But use of the gas for military purposes would be a breach of its undertakings under the deal.
More than 230,000 people have been killed in Syria since the conflict began in March 2011 with anti-government protests before spiralling into a complex multi-front war.