Last September, the Organisation for the Prohibition of Chemical Weapons (OPCW) received an initial disclosure from the Syrian government of its chemical weapons programme, the first time the country had made a formal declaration. Syria also signed the Chemical Weapons Convention (CWC) and agreed to the destruction of its chemical weapons.
The OPCW has yet to release the details of the declaration, leaving the size of Syria's arsenal subject to speculation.
But experts believe the stockpile, considered to be one of the world's largest, contains the blister agent sulphur mustard, the nerve agent sarin, and the more potent and persistent nerve agent VX.
Following a deadly chemical weapons attack in Damascus on 21 August 2013, the United States and Russia agreed a plan with Syria to remove and destroy its chemical weapons by mid-2014.
Weapon development and delivery
The US stated in 2002 that Syria had a "long-standing chemical warfare programme", which was first developed in the 1970s. A recent report from the US Congressional Research Service said Syria probably began stockpiling chemical weapons in 1972 or 1973, when it was given a small number of chemicals and delivery systems by Egypt before the 1973 Arab-Israeli war.
Damascus started acquiring the materials and knowledge necessary to produce chemical weapons in the 1980s, reportedly with the help of the Soviet Union. Equipment and chemicals are also thought to have been procured from European companies.
In 2011, the US director of national intelligence concluded that Syria remained "dependent on foreign sources for key elements" of its chemical weapons programme, including precursor chemicals, which are generally dual-use chemicals that can be combined to produce blister or nerve agents.
Before the uprising, the Syrian Scientific Studies and Research Centre (CERS) was believed to run at least four chemical agent manufacturing plants - at Dumayr, Khan Abou, Shamat and Furklus - and operate additional storage sites dispersed across the country in some 50 different towns and cities.
The exact size of Syria's chemical weapons arsenal is not known, but in June 2012, Israel Defense Forces (IDF) Deputy Chief of Staff Maj Gen Yair Nave described it as "the largest in the world".
According to a French intelligence assessment published in September 2013, Damascus has more than 1,000 tonnes of chemical agents and precursor chemicals, including:
- Several hundreds of tonnes of sulphur mustard
- Several hundreds of tonnes of sarin
- Several tens of tonnes of VX
According to the White House, the Syrian military has access to "thousands of munitions" that can be used to deliver chemical warfare agents, including a variety of long- and short-range ballistic missiles, aerial bombs and artillery rockets.
Since the beginning of the uprising, ammunitions carrying lesser volumes are believed to have been developed for more focused and local tactical use.
Regarding the chain of command, the French intelligence assessment said the section of the Syrian military responsible for filling munitions with chemical agents and for security at storage sites - "Branch 450" of the CERS - was staffed only by members of the president's minority Alawite sect and was "distinguished by a high level of loyalty to the regime".
"Bashar al-Assad and certain influential members of his clan are the only ones permitted to give the order for the use of chemical weapons. The order is then transmitted to those responsible at the competent branches of the CERS," it added. "At the same time, the army chiefs of staff receive the order and decide on targets, the weapons and the toxic agents."
- Human Rights Watch says 330mm surface-to-surface rockets were used to attack Zamalka in Eastern Ghouta on 21 August. At least four strike sites have been found.
- From remnants of the weapons found, HRW has reconstructed the characteristics of the rocket. It says it was capable of carrying up to 60 litres of chemical nerve agent.
- The payload of the rocket consisted of a large, thin-walled container. A small explosive charge at the front would detonate on impact and rupture the skin, dispersing the chemicals inside.
Syria is widely believed to possess large quantities of the blister agent sulphur mustard. The term "mustard gas" is commonly used to describe the agent, but it is liquid at ambient temperature.
Sulphur mustard sometimes smells - like garlic, onions, or mustard - and sometimes has no odour. It can be clear to yellow or brown.
People can be exposed through skin contact, eye contact or breathing if it is released into the air as a vapour, or by consuming it or getting it on their skin if it is in liquid or solid form. It causes blistering of the skin and mucous membranes on contact.
Though exposure to sulphur mustard usually is not fatal, there is no treatment or antidote to mustard which means the agent must be removed entirely from the body.
Syria started to produce tube and rocket artillery rounds filled with mustard-type blistering agents in 1993, presumed to be the first weaponisation of its kind.
Sarin is a neurotoxic organophosphorus compound that is highly toxic and lethal.
It is considered 20 times as deadly as cyanide and is impossible to detect because it is a clear, colourless and tasteless liquid that has no odour in its purest form. It can also evaporate and spread through the air.
As with all nerve agents, Sarin inhibits the action of the acetylcholinesterase enzyme, which deactivates signals that cause human nerve cells to fire. This blockage pushes nerves into a continual "on" state. The heart and other muscles - including those involved in breathing - spasm. Sufficient exposure can lead to death via asphyxiation within minutes.
Sarin, like VX, is believed to be stocked in a "binary manner" by the Syrian military. This means it is kept as two distinct chemical precursors, which are combined just before use, either manually or automatically inside a weapon when launched.
According to a report by UN chemical weapons inspectors, there is "clear and convincing evidence" that surface-to-surface rockets containing sarin were fired at suburbs to the east and west of Damascus in an attack on 21 August that killed hundreds of people.
According to US, British, French and Israeli officials, there is also evidence that Syrian government forces used sarin against rebels and civilians on several previous occasions.
French intelligence said analysis of samples taken from the northern town of Saraqeb and the Damascus suburb of Jobar in April showed that munitions containing sarin had been deployed. However, doubts have been expressed about the chain of custody of those samples as they travelled from their original locations in Syria to laboratories in other countries.
VX, another neurotoxic organophosphorus compound, is the most toxic known chemical warfare agent - about 10 times more toxic than sarin.
It is an oily liquid that is amber in colour, and is odourless and tasteless. Once it is released into the air, people can be exposed through skin contact, eye contact or inhalation.
Symptoms appear within a few seconds after exposure to the vapour form of VX, and within a few minutes after exposure to the liquid form. As with sarin, VX attacks the nervous system.
VX evaporates about as slowly as motor oil and can persist for a long time under average weather conditions, unlike sarin. It therefore poses both a short- and long-term threat.