The US military said Monday it is "concerned" about reports that Islamic State jihadists used chlorine gas in an attack against Kurdish forces, but it could not confirm the account.

Iraq's autonomous Kurdistan government said Saturday that an analysis of soil and clothing samples showed that the IS group employed chlorine gas in a car bomb attack on January 23.

"We are certainly concerned about it. We have not been able to independently confirm it," said Pentagon spokesman Colonel Steven Warren.

"We have seen what the Kurds have to say. We have no reason not to believe them," he told reporters.

The alleged attack was another example of the "brutality" of the IS extremists and could signal their "desperation" as they come under mounting pressure on the battlefield, Warren said.

The use chlorine gas in homemade bombs is not new in the conflict zone. The Syrian regime of President Bashar al-Assad has frequently used chlorine in attacks on opposition forces.

"This use of industrial chemicals is in the mix in that region of the world," Warren said.

Supplies of chlorine are readily available at water treatment plants that the IS group would have access to in captured territory, said Daryl Kimball, executive director of the Arms Control Association, a non-profit group based in Washington.

Creating chlorine-filled explosives violated international law prohibiting chemical weapons, Kimball told AFP.

"This is yet another escalation by Daesh forces in their reign of terror," he said, using the Arabic term for the IS group.

The Assad regime's use of chlorine gas apparently had "inspired" the IS group, he said.

Syria agreed to give up its arsenal of lethal nerve agents under an agreement brokered by the United States and Russia. International monitors verified the removal of the substances last year.

Without the disarmament deal, the IS group might have seized control of some of the chemical weapons and would not have hesitated to use them, Kimball said.

"That's why it was so important to get that material out of there," he said.

Car bombs and roadside bombs are easy to rig with chlorine canisters but the chemical agent tends not to be effective as a weapon, only affecting small numbers of people. The weapon, however, carries a psychological effect.

Doctors and witnesses have reported people exposed to chlorine gases in recent attacks in Iraq briefly showing symptoms such as vomiting and respiratory problems.

It is unclear, however, whether anyone has died as a result of exposure to chlorine used in an IS attack.

The first time chlorine was used on the battlefield was about 100 years ago in World War I, when German forces fired canisters with chlorine gas.