CIA chief John Brennan
CIA chief John Brennan on CBS's 60 minutes news programme

The Central Intelligence Agency has warned that Islamic State fighters have used chemical weapons in the battlefield and that they have the capability of making small quantities of chlorine and mustard gas. US officials had last year suggested that Isis may have obtained sulphur mustard in Syria, despite the government declaring that all of its stockpiles had been destroyed under a disarmament deal.

CIA Director John Brennan also warned of the possibility that the militant group could seek to export the weapons to the West for financial gain. He said: "We have a number of instances where ISIL [another name for the Islamic State] has used chemical munitions on the battlefield." Excerpts of Brennan's interview with CBS's 60 Minutes news programme due to be aired on 14 February was released on 11 February.

The intelligence officer also warned that Daesh could seek to export the chemical weapons to the West for financial gain. "I think there's always the potential for that. This is why it's so important to cut off the various transportation routes and smuggling routes that they have used," he added.

On whether there were "American assets on the ground" searching for possible chemical weapons caches or labs, he said: "US intelligence is actively involved in being a part of the efforts to destroy ISIL and to get as much insight into what they have on the ground inside of Syria and Iraq."

News agency AFP noted that this is not the first time the issue of chemical weapons being used by Isis militants was raised. It said National Intelligence Director James Clapper told a Congressional committee on 9 February: "Isil has also used toxic chemicals in Iraq and Syria, including the blister agent sulphur mustard."

He said it was the first time an extremist group had produced and used a chemical warfare agent since Japan's Aum Supreme Truth cult carried out a deadly sarin attack during rush hour in the Tokyo subway in 1995. Officials in the autonomous Iraqi region of Kurdistan had said last year that blood tests had shown that Isis fighters had used mustard agent in an attack on Kurdish peshmerga forces in August. It claimed that 35 peshmerga fighters were exposed and dome had sought treatment abroad.

Last year, the Syrian American Medical Society reported it received 50 patients showing signs of chemical exposure. Similarly, Medecins San Frontieres claims that it had treated four members of a family believed to have been exposed to a chemical agent. The Shami Front, a Syrian rebel group has also claimed that half of the mortars and artillery round which hit Marea contained sulphur mustard.