Volunteers wearing gas masks during a class on how to respond to a chemical attack, in the northern Syrian city of Aleppo on September 15, 2013
Paris (AFP) - Mustard gas, used during fighting in Syria according to the global chemical weapons watchdog, first terrorised battlefields during World War I.
Most recently, it was used on August 21 in Marea, a town in the northern province of Aleppo, a source at the Organisation for the Prohibition of Chemical Weapons (OPCW) told AFP on Friday.
"We have determined the facts, but we have not determined who was responsible," the source said.
But the Britain-based Syrian Observatory for Human Rights and activists from Marea have accused the Islamic State jihadist group of being behind the attack.
A so-called vesicant agent officially identified as bis-(2-chloroethyl)sulfide, mustard gas has also been dubbed Yperite because it was first used near the Belgian city of Ypres in July 1917 by the German army.
German troops had already experimented with a chlorine-based gas in the area on April 22, 1915.
They called mustard gas "LOST" in reference to the names of the chemists at the Bayer group who developed the gas.
Masks made to protect against chlorine agents were useless because mustard gas penetrated them and attacked the skin. French chemists subsequently developed a much more effective form which is said to have been decisive in winning the second Battle of the Marne in 1918.
Although the gas killed relatively few people in a war that caused millions of deaths, it marked soldiers because of its terrible effects and the collective terror it was responsible for.
An oily yellow almost liquid-like substance that smells like garlic or mustard, it does not need to be inhaled, and settles on exposed surfaces where it can remain active for several weeks, according to chemist Jean-Claude Bernier.
It causes the skin to break out in painful blisters, irritates eyes and causes eyelids to swell up, temporarily blinding its victims.
Internal and external haemorrhaging then results and destroys the lungs. Victims typically die within four to five weeks of a pulmonary oedema as liquid accumulates in the respiratory system.
Mustard gas can be encased in warheads fired from cannons or dropped from aircraft, and was used many times during World War I.
After that it was used in Russia in 1919, in Morocco by the French between 1923 and 1926, in Libya by the Italians in 1930, according to the World Health Organisation.
It was also used in Xinjiang, China, by the Japanese in 1934, and in Ethiopia by the Italians between 1935 and 1940, the WHO says.
Japanese forces used chemical weapons again in China between 1937 and 1942, including mustard gas, and Iraq used it against Iran between 1980 and 1988.
Iraq also used it against the Kurdish village of Halabja in March 1988, killing almost 5,000 people.
The use of chemical weapons, but not their development, was outlawed in 1925 by the Geneva Protocol.
It was not until the signing in 1993 of the Paris Convention and its taking effect on April 29, 1997 however, that the complete development, manufacturing, storage and use of chemical weapons was banned.