"We have a number of instances where ISIL has used chemical munitions on the battlefield," CIA Director John Brennan said in excerpts of an interview with CBS News to be broadcast on Sunday, referring to "Islamic State" by an alternate acronym.

There have been previous claims that IS had used chemical weapons against Kurdish fighters in Iraq and Syria. Iraqi Kurdish claims were later verified by testing.

There has also been speculation the group had either captured chemical weapons stocks in Syria or had gained access to precursors agents to make chemical weapons.

But Brennan's comments add greater certainty to the terror group's production capability.

"There are reports that ISIL has access to chemical precursors and munitions that they can use," Brennan said, adding that there was "always potential" for IS to try to sneak chemical weapons into the West to carry out an attack.

"This is why it's so important to cut off the various transportation routes and smuggling routes that they have used," he said.

Brennan's comments come two days after James Clapper, the director of national intelligence, told a congressional hearing Islamic State had used chemical weapons including mustard gas.

cw/sms (AFP, CBS)