A United States official said there was a growing belief within the US government that ISIS started to make and use crude chemical weapons in Iraq and Syria.
The US government identified at least four occasions in both countries, where ISIS militants used mustard agents, a US security agent said, adding that chemical was used in powder form. The mustard powder were added to normal weapons such as, mortar rounds. If these weapons explode, contact causes blistering of the skin, eyes and respiratory tract.
The news came after images of Kurdish soldiers allegedly injured by chemical weapons as they were fighting with ISIS were released. The images showed burns and blisters of the soldiers allegedly caused by mustard powder.
It is allegedly said by the US that ISIS was producing these chemical weapons on its own in a cell, just used for making weapons.
'We've seen them use it on at least four separate occasions on both sides of the border - both Iraq and Syria," the US official said.
The official said that intelligence community explains three possible ways for how ISIS acquired the chemical agents. According to the intelligence community, the most probable one is that they are developing it.
"We assess that they have an active chemical weapons little research cell that they're working on to try and get better at it," the official said.
The earlier alternate theories was that ISIS may have found the weapons, cached in Iraq an Syria. However, the official said it was unlikely to find chemical weapons in Iraq as the US military would have found them during the military campaign which lasted about a decade in the country.
It was also unlikely to acquire the chemical agent from Syria because the Syrian regime handed over its stockpile under the threat of US air strikes in 2013.
According to the official, the US believes that they are producing the mustard agent, which is not a complex chemical to make.
The US government continues to investigate claims of using chemical weapons in Iraq and Syria, but the official who spoke to BBC said intelligence agencies believe there are evidences backing these chemical weapons claims.
The official asked for anonymity because he was not authorised to talk about the issue publicly.
The US military conducted tests on ISIS mortar fragments from fighting in Iraq and the results showed traces of chemical agents. US Brig. Gen. Kevin Killea said that the US had found traces of chemical agent sulphur mustard on mortars used in the fighting between ISIS and Kurdish forces in Northern Iraq. He added that the tests were not definite, final testing must have been conducted. Killea classified sulphur mustard as a Class 1 chemical agent, which is mostly used inside of chemical warfare.
The United Nations launched investigation on using chemical weapons during Syrian civil war to determine individuals, groups and governments are involved in.
In March, Iraqi officials showed a footage confirming that ISIS militants were using chlorine gas in roadside bomb attacks in Iraq. The videos showed plumes of orange smoke into the air after the explosion.
Since late last year, multiple reports have been revealed that ISIS was using chlorine gas but the officials had not been sure till the footage showed its use.
Haider Taher, from the Iraq Bomb Disposal Team, said the military defused dozens of devices containing chlorine gas during the figthing with ISIS.
"They have resorted to this new method," he told the BBC. "They're putting chlorine inside these homemade roadside bombs, which is toxic for those that inhale it."
However, experts explained that the bombs, containing small amount of chemical agent concentrations, were designed to cause fear and panic rather than harm.