But eyewitness accounts of that attack, in which six rebels died and which were reported at the time by the Associated Press described “white smoke” pouring from shells that “smell[ed] … like hydrochloric acid”.

The suggestion that one of the sarin-positive samples may have originated in Homs has added to the growing confusion surrounding the claims made with different degrees of caution by the Israeli, French, UK and US governments in recent days.

According to the US and UK governments, “miniscule” samples recovered by opposition sources and passed on to western intelligence agencies have shown traces of sarin. No other agents have been mentioned.

While the contradictions between the eyewitness accounts and traces of sarin in the samples may well be attributable to the confusion of battle, it underlines the uncertainties around the claims, which have included questions about whether some of the videotaped symptoms are consistent with sarin exposure.