Mounting evidence suggests Islamic militants in Iraq are weaponizing chlorine, a widely available industrial chemical that can be used as a "choking agent" on the battlefield.
Reports of chlorine found in improvised explosive devices, mortars, rockets and missiles have cropped up in recent months and suggest that the Islamic State militants are trying out new tactics, chemical weapons experts say.
IS extremists have used chlorine in the past, mainly in blunt, terrorist-style attacks. But now some experts say the group is trying to advance its technology for deploying the chemical in combat operations.
"They seem to be playing around with dissemination technology, however crude it may be," said Jean Pascal Zanders, a chemical weapons expert and former senior research fellow at the European Union's Institute for Security Studies.
"If the battles continue, one could probably see a gradual development whereby the weapon evolves from something of a stand-alone attack, perhaps with a terrorist intent, to the potential use of IEDs in a more warfare-like type of operation," Zanders said in an interview.
In its natural form, chlorine is a yellow-green gas that is potentially fatal in high concentrations. In lower density, it can damage the lungs and cause choking as well as vomiting, nausea, dizziness and unconsciousness.
It is less lethal than some other chemical weapons such as Sarin or mustard gas, but it is also far easier to obtain because it has many legitimate industrial uses, including for sanitizing drinking water.
U.S. military officials say they have been unable to "independently verify" that Islamic State militants in Iraq are using chlorine, but they are aware of the reports.
"We know that this use of industrial chemicals is in the mix in that region in the world. … It's not weaponized chemicals in the traditional sense. It's almost homemade," said Army Col. Steve Warren, a Pentagon spokesman.
There have been no reports of chlorine attacks on the roughly 2,800 U.S. troops currently deployed to Iraq, military officials say.
The latest report of a chlorine attack by the Islamic State militants, also known as ISIS or ISIL, came in mid-March when the Kurdish government in Iraq said tests results confirmed chlorine was used in a suicide truck bomb that targeted Kurdish peshmerga fighters in January.
They also said a more recent bomb attack in northern Iraq appeared to involve chlorine when several military police experienced unconsciousness and vomiting.
The Islamic State does not operate any chemical production facilities, Zanders said.
Instead, they are seizing chlorine from water treatment plants and industrial chemical facilities in the territories they control in Iraq and Syria, he said.