Islamic State group fighters fired missiles containing toxic gas at Kurdish forces in Syria last month, the forces and a monitoring group said.

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 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," the statement 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 IS 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.

- Iraq chlorine probe -

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 analysed, but one expert told the newspaper he was "certain it is chlorine."

IS 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 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 in a rebel-held Damascus suburb.

The Syrian opposition and much of the international community blamed the attack on the Damascus government.

It 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.

Damascus 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.