ISIS has is making and using crude chemical weapons and has used them on at least four occasions on both sides of the Iraq-Syria border, according to a BBC report.

A US official told the news agency that there is a growing belief within the US government that the Islamic State militant group is making and using crude chemical weapons in Iraq and Syria.

It says the US has identified four occasions where ISIS used mustard gas, which is being used in powder form. The BBC says it has seen evidence backing these claims.

It is believed that ISIS has a dedicated group building these weapons. “The mustard agent was probably being used in powder form and packed into traditional explosives like mortar rounds,” the US official said. When these weapons explode the mustard-laced dust blisters those who are exposed to 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.

Knowledge to make the mustard agent is widely available, and it is not a complex chemical to produce. The alternative theories are that IS militants found chemical weapons caches in Iraq or in Syria.

Both the Al-Assad regime and the Syrian rebels have reportedly used chemical weapons in the five-year-old conflict, despite a chemical weapons “handover” that began in October 2013.

Last month a US general said tests on IS mortar fragments from fighting in Iraq found traces of Sulphur mustard. The type of chemical found is rarely used outside chemical warfare, he said.