Syria's arsenal of chemical weapons has existed for several decades and is considered one of the biggest in the Middle East, but its exact makeup and size remain guesswork as few facts have emerged.
UN investigators said Tuesday said they had "reasonable grounds" to believe both sides in Syria have used chemical weapons, while French Foreign Minister Laurent Fabius said that Paris had solid proof that President Bashar al-Assad's regime was using sarin gas.
In the United States the White House said more evidence was necessary.
The Syrian regime acknowledged for the first time on July 23, 2012, that it had chemical weapons and threatened to use them in case of a Western military intervention, but never against the Syrian population.
The government and the armed opposition accuse each other of having used chemical weapons during the conflict which has raged for more than two years.
Syria is one of the few countries not to have signed the Chemical Weapons Convention and is believed to have a large stockpile of sarin and other nerve gases.
The Syrian programme was launched in the 1970s with the help of Egypt and the then Soviet Union.
In the 1990s Russia provided support, followed by Iran since 2005, according to the Nuclear Threat Initiative (NTI), an independent organisation tracking data on arms of massive destruction.
An analyst at the non-proliferation and disarmament programme of the International Institute for Strategic Studies (IISS), says Syria has the biggest chemical weapons programme in the Middle East, created at the start with the goal of counterbalancing Israel's nuclear programme.
The analyst says important information on the programme has been collected following the defection of several Syrian military officers, but that the information is "far" from complete.
According to an expert at the Monterey Institute for International Studies in the United States, Syria has "hundreds of tonnes" of diverse chemical agents.
"Their armoury of chemical agents is quite strong," according to a French specialist in chemical weapons at the Foundation for Strategic Research.
"The Syrians have managed to master the synthesis of organophosphorus, that's the last generation, the most efficient and most toxic of chemical weapons. In this family, one finds sarin and VX, as well as ... mustard gas," he said in July 2012.
On January 30, the Israeli air force bombarded a site of ground-to-air missiles and an adjacent military complex near Damascus suspected of holding chemical agents, with Israel saying it feared the transfer of chemical weapons to the Lebanese Shiite movement Hezbollah, according to a US official.
According to The New York Times, the raid could have damaged Syria's main research center into biological and chemical weapons.
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