New questions have emerged over the source of the soil and other samples from Syria which, it is claimed, have tested positive for the nerve agent sarin, amid apparent inconsistencies between eyewitness accounts describing one of the attacks and textbook descriptions of the weapon.

As questions from arms control experts grow over evidence that the Assad regime has used chemical weapons on a limited scale on several occasions, one incident in particular has come under scrutiny.

While the French, UK and US governments have tried to avoid saying where the positive sarin samples came from, comments by officials have narrowed down the locations to Aleppo and Homs.

Last week the Obama administration suggested that Syrian government forces may have used the lethal nerve gas in two attacks. Opposition fighters have accused regime forces of firing chemical agents on at least four occasions since December, killing 31 people in the worst of the attacks.