It has long been suspected and now an official investigation reportedly concludes that government troops were behind two toxic gas attacks in Syria.

A confidential report – by the UN and the global chemical weapons watchdog – is also believed to say that ISIL militants have used sulfur mustard gas.

The inquiry focused on nine attacks in seven areas of Syria, where a separate OPCW fact-finding investigation had already determined that chemical weapons had likely been used.

The latest inquiry found there is sufficient information to conclude that Syrian Arab Air Force helicopters dropped devices that released toxic substances in Talmenes in 2014 and in Sarmin last year – both in Idlib province. The cases involved the use of chlorine.

It also determined there was sufficient information to conclude that ISIL militants were the “only entity with the ability, capability, motive and the means to use sulfur mustard gas in Marea on 21 August 2015.”

The Syrian mission to the UN did not immediately respond to a Reuters request for comment.

The findings set the stage for a UN Security Council showdown over whether sanctions should now be imposed. The 15-member Council is due to discuss the report next week.

Secretary-General Ban Ki-moon said the report would be made public after that meeting.