The UN chemical weapons watchdog Friday confirmed with "utmost confidence" that mustard gas was used in Syria in August during fighting between rebels and jihadists and "likely" killed a child.

Experts from the Organisation for the Prohibition of Chemical Weapons also found toxic chemicals, including chlorine, were likely used as a weapon in an attack in Idlib province in March, the OPCW said in statement.

Three reports have been sent by the head of the OPCW to the body's 192 members after separate missions to investigate incidents in Syria.

In one attack in the town of Marea in Aleppo province on August 21, the OPCW team investigated after "a non-state actor had allegedly used a chemical weapon."

They collected samples and "interviewed two individuals affected by exposure" as well as the doctors that treated them.

"In this case, the team was able to confirm with utmost confidence that at least two people were exposed to sulphur mustard, and that it is very likely that the effects of this chemical weapon resulted in the death of an infant," the OPCW statement said.

OPCW sources told AFP late Thursday investigators had for the first time in the four-year war confirmed the use of deadly mustard gas in Syria.

First used in battle in World War I, the gas 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.

While the OPCW's mandate is not to apportion blame, activists on Friday accused militants from the Islamic State group of using the gas as part of its sustained campaign to capture Marea.

Doctors Without Borders (MSF), which operated a nearby field clinic, treated four members of a single family for "symptoms of exposure to chemical agents" after the Marea attack.

Residents told MSF they saw a "yellow gas" when a mortar round hit their house.

- New chlorine attack -

In a separate investigation, OPCW experts probed allegations that toxic chemicals were unleashed in March in northwestern Idlib.

The team "concluded that the alleged incidents likely involved the use of one or more toxic chemicals - including chlorine - as a weapon."

Human Rights Watch had accused the regime of Syrian President Bashar al-Assad of dropping barrels filled with chlorine in the rebel-held area during six attacks from March 16 to 31.

However, in a third incident in which the Syrian government said its soldiers had been exposed to toxic chemicals in Jobar on the eastern edge of Damascus on August 29, the OPCW "could not confidently determine that a chemical was used as a weapon."

In September 2014, the OPCW confirmed that chlorine was used as a chemical weapon "systematically and repeatedly" in villages in northern Syria earlier in the year. It cited attacks in the villages of Talmanes, Al-Tamana and Kafr Zeita.

Now a special OPCW mission has been set up to investigate who is behind the deadly gas attacks.

Under a deal hammered out in 2013 between Russia and the United States following a sarin gas attack on the outskirts of Damascus in which hundreds died, the regime pledged to hand over all its toxic weapons to the OPCW for destruction.

Assad's government had had stockpiles of more than 19 tonnes of mustard gas.

But activists said IS had probably managed to get hold of the gas smuggling it via Turkey or Iraq.

Mustard gas: A legacy of WWI
Paris (AFP) Nov 6, 2015 - 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.