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.
A BBC team on the Turkey-Syria border has seen evidence backing these claims.
The US believes the group has a cell dedicated to building these weapons.