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, a US official has told the BBC.

The US has identified at least four occasions on both sides of the Iraq-Syria border where IS has used mustard agents, the official said.

The official said the chemical was being used in powder form.

The US believes the group has a cell dedicated to building these weapons.

“They’re using mustard,” the individual said of IS. “We know they are.”

The mustard agent was probably being used in powder form and packed into traditional explosives like mortar rounds, the official said.

“We’ve seen them use it on at least four separate occasions on both sides of the border – both Iraq and Syria.”