Human Rights Watch, the New York-based rights monitor, has released a report with evidence that strongly suggests Syrian government forces used toxic chemicals in several barrel-bomb attacks in Idlib province beween March 16 and 31, 2015.

Rescue workers reported that the attacks affected 206 people, including 20 civil-defence workers. One attack reportedly killed six civilians, including three children.

In a closed-door meeting on April 17, the UN Security Council heard first-hand accounts from Syrian doctors of the same attack.

Evidence presented during the meeting included a video showing doctors trying to save the lives of three childrens  following the attack as they coughed and struggled to breathe due to liquid accumulation in their lungs.

The attacks violated the Chemical Weapons Convention, which Syria ratified in October 2013, and a UN Security Council resolution, Human Rights Watch said.

Hamish De Bretton, one of the world's leading chemical-weapons experts, told Al Jazeera that he trained people in Syria during September 2014 to collect samples of the attacks which would be admissible in the ICC and UN.

"Last week I was on the Syrian border and was handed samples from the March chlorine barrel bomb attacks in Sarmin which proved positive for chlorine. This attack killed six people and injured 85 and this evidence was presented to the UNSC last night," he said.

"Chemical weapons is a physiological weapon and creates huge fear even in Syria. People tell me they can hide from bombs and bullets but not from gas.

Image of refrigerant canister found by Syria Civil Defence among the remnants of a barrel bomb dropped by Syrian government forces on Binnish on March 24, 2015 [Syrian Civil Defence]

De Bretton, who works with several charities including Union of Syrian Medical Relief Organisations (UOSSM) and Syria Relief, instructs doctors and and first responders to treat chemical casualties and to advise civilians on how to avoid becoming casualties.

He told Al Jazeera that not all hospitals are equipped to look after patients who have been affected by chemical attacks.

The Syrian regime has endless amounts of chlorine despite ostensibly giving up all its chemical-weapons arsenal, he said.

Below are some of the chemical attacks recorded and investigated by Human Rights Watch or the UN.

2015 - March 16 and March 31: Human Rights Watch initiated further investigation into the alleged use of chemical weapons in Idlib province. Evidence showed use of chlorine gas in most areas.

2014 - September: a fact-finding mission appointed by the Organisation for the Prohibition of Chemical Weapons (OPCW) found "with a high degree of confidence, that chlorine was used as a weapon systematically and repeatedly on towns in northwestern Syria (Hama, Aleppo, Idlib).

Human Rights Watch investigation into the April 2014 attacks found evidence that strongly suggested that the chlorine-gas cylinders were contained in barrel bombs dropped by government helicopters.

2013 - April: the Syrian Observatory for Human Rights (SOHR) said that the Syrian army dropped two gas bombs on rebel-controlled Aleppo, killing two people and wounding 12. 

August: Available evidence strongly suggests that Syrian government forces were responsible for chemical weapons attacks on two Damascus suburbs on August 21, 2013.These attacks, which killed hundreds of civilians including many children, appeared to use a weapons-grade nerve agent, most likely sarin.

Syrian opposition activists claimed that a large-scale chemical-weapons attack occurred at the suburbs of the Ghouta region, where Syrian forces had been attempting to expel rebel fighters.

Reports said that thousands of victims of the attack have been counted in the Damascus suburbs, whose symptoms were typically body convulsions, foaming from mouths, blurry vision and suffocation.

Although the number of victims has not been clarified yet, it is estimated to exceed 1,000.

March: Alleged chemical weapons attacks were reported in the Khan al-Assel neighbourhood of Aleppo and the Damascus suburb of al-Atebeh. About 25 people reportedly were killed and dozens more injured.

Assad's government accused opposition fighters of using chemical weapons in the fighting there.

2012 - December 23: The first allegation of  chemical weapons use was reported in Homs.  Seven people were allegedly killed in Homs by a "poisonous gas" used by government forces. The coverage included reports of side effects such as nausea, relaxed muscles, blurred vision and breathing difficulties.

Source: Al Jazeera