Syria maintains an arsenal of chemical weapons that is considered one of the biggest in the Middle East, but its makeup and size remain guesswork as few facts have emerged.

Britain, France and the United States have accused the Syrian army of limited use of chemical weapons against opposition forces since an uprising broke out against President Bashar al-Assad's rule in March 2011.

Russia, which backs the government, said earlier this month that it had proof that rebel fighters had employed Sarin nerve gas in March near Aleppo, in northern Syria.

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 28-month conflict.

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 weapons of mass 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, launched 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 a specialist at the Monterey Institute for International Studies in the United States, Syria has "hundreds of tonnes" of diverse chemical agents.

A French specialist at the Foundation for Strategic Research said: "Their armoury of chemical agents is quite strong.

"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 bombed a ground-to-air missile battery and adjacent military complex near Damascus suspected of holding chemical weapons, with Israel saying it feared their transfer to Lebanese Shiite militant group Hezbollah, according to a US official.

According to The New York Times, the raid could have damaged Syria's main research centre into biological and chemical weapons.

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