An Islamic State flag is seen in this picture illustration taken February 18, 2016. REUTERS/Dado Ruvic/Illustration/File Photo

PARIS (Reuters) - Islamic State militants returning from Syria could carry out mustard gas attacks after learning how to use the toxic substance in battle zones, a senior official from the global chemical weapons watchdog warned on Wednesday.

An inquiry by the United Nations and the Organization for the Prohibition of Chemical Weapons (OPCW) concluded in October that Syrian government forces were responsible for chlorine gas attacks and that Islamic State militants had used mustard gas.

“It seems that one of the dangers that we need to face and have a response for - since Islamic State has learnt how to make mustard gas - is that sadly one of the people who learnt how to do it comes back to one of our countries and helps carry out an attack like this,” Philippe Denier, director at the verification division of the OPCW told a defense conference in Paris.

Western officials are increasingly worried that Islamic State, which is fighting to defend territory it seized in Syria and Iraq, will call on its followers and jihadists returning from the region to increase attacks in the West.