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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.

"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."

When these weapons explode the mustard-laced dust blisters those who are exposed to it.

The official said the intelligence community believes the most plausible explanation is that they are manufacturing 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 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.