The Islamic State group fired chemical weapons against Kurdish forces in Syria and Iraq last month, Kurdish fighters and weapons experts have said.

The Syrian Observatory for Human Rights, a UK-based group that reports on the war using an activist network on the ground, said it had also documented the use of poison gas by ISIS in an attack on a village near Tel Brak on June 28.

The Conflict Armament Research (CAR) group and Sahan Research in a statement on Friday said that the extremist group targeted Iraqi Kurdish Peshmerga with a projectile filled with a chemical agent on June 21 or June 22.

The chemical used in the Iraq attack had characteristics and clinical effects “consistent with a chlorine chemical agent”, the groups said.

Redur Xelil, the YPG spokesman, said on Saturday that the type of chemical used had not been definitively determined.

Those attacks, at Tel Brak and Hasakah, occurred in late June and appeared to involve shells or small rockets containing an industrial chemical sometimes used as a pesticide, the investigators said.

Kurdish forces are playing a vital role in the fight against ISIS in both Syria and Iraq. YPG forces say they’ve captured gas masks from ISIS fighters in recent weeks. The Observatory added that it received information about the gas being fired into Hasaka city, but gave no further details.

“Soon we should have an exact composition of the chemical in this projectile, but I am certain it is chlorine”, Gregory Robin from Sahan Research said as quoted by the newspaper.

It said soldiers exposed to the gas “experienced burning of the throat, eyes and nose, combined with severe headaches, muscle pain and impaired concentration and mobility”.

Kurdish forces are enjoying an important position in the struggle towards Islamic State in each Syria and Iraq.

Some have been confirmed by the worldwide chemical weapons watchdog but it did not say who carried them out. The device was crude, hence the failure to explode, but it was leaking a chemical that caused several Kurdish fighters to get sick.

According to investigators in the field, Kurdish officials and physical evidence, it is likely that ISIS is using toxic materials in their attacks directed at Kurdish positions along the vast front, according to the New York Times.

Reports of poison gas attacks by the ISIS could not be independently verified by Reuters.

The United States, along with Britain and France, accuse Syrian president Bashar al-Assad’s regime of using chlorine gas against civilians.