Director of National Intelligence, James Clapper, said “sulphur mustard” has been used in an alleged Isis attack in Syria and there are “numerous allegations” of Isis’ use of chemical weapons in both countries, while speaking on Capitol Hill before the Senate Armed Services on Tuesday.

In a report accompanying his testimony Mr Clapper states: “We asses that non-state actors in the region are… using chemicals as a means of warfare."

He said an investigation “into an alleged [Isis] attack in Syria in August" concluded "at least two people were exposed to [the blister agent] sulphur mustard.

"We continue to track numerous allegations of [Isis`] use of chemicals in attacks in Iraq and Syria.”

Mr Clapper is quoted by Fox News as saying this is the first time an extremist group has produced and used a “chemical warfare agent in an attack since Aum Shrinrikyo used sarin in Japan in 1995,” referring to a subway attack in Tokyo that year.

Mr Clapper made the revelations amid an annual assessment of threats facing the US, which ranged from cyber security to drugs.

In the assessment, dubbed a “litany of doom” by Mr Clapper, he described Isis as “the pre-eminent global terrorist threat,” adding the group’s “estimated strength worldwide exceeds that of al-Qaida.”

The head of the Defence Intelligence Agency, Lt Gen Vincent Stewart, who also testified, said Isis "will probably attempt to conduct additional attacks in Europe, and attempt to direct attacks on the US homeland in 2016", CNN reports.

Late last year, reports surfaced suggesting Isis was “aggressively pursuing the development of chemical weapons”, creating a team dedicated to research and experiments.

Iraqi and US intelligence officials raised concerns that a large area controlled by extremists, since the group overran parts of Iraq and Syria last year, has left authorities largely in the dark about Isis activities.

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