Syria\'s chemical weapons stockpile, which Damascus has acknowledged for the first time, is decades old and among the biggest in the Middle East, but experts are divided over its exact nature. President Bashar al-Assad\'s regime caused a global uproar Monday when it vowed to use its chemical weapons if attacked by outsiders, although not against its own people. But public data about the stockpile is scarce, as Syria is one of the few countries that has not signed the Chemical Weapons Convention. Damascus has however signed the Geneva protocol, which bars the first use of chemical and biological weapons, though it does not make stipulations about production, storage and transfer. \"We are closely following information about Syria... but we can\'t say more without sending inspectors on the ground,\" Michael Luhan, a spokesman for the Organization for the Prohibition of Chemical Weapons, told AFP. Intelligence services also have little to say on the subject. \"Syria\'s well-established chemical warfare program includes a stockpile of nerve agent, which can be delivered by aircraft or ballistic missiles,\" Defense Intelligence Agency chief Lieutenant General Ronald Lee Burgess said in March 2011 testimony before a Senate panel. \"Syria continues to seek chemical warfare-related precursors and expertise from foreign sources.\" Syria is stockpiling \"hundreds of tons\" of various chemical agents, according to Leonard Spector of the Center for Nonproliferation Studies at the Monterey Institute of International Studies. \"Their panoply of chemical agents is quite robust,\" said Olivier Lepick of the Foundation for Strategic Research think tank in Paris. \"They have successfully mastered the synthesis of organophosphorus compounds -- the latest, most efficient and most toxic generation of chemical weapons.\" This family of compounds includes sarin and VX nerve agents, as well as older agents such as mustard gas, a mix of sulfur dioxide and ethylene. Syria\'s program was launched in the 1970s with help from Egypt and later from the former Soviet Union. In the 1990s, Russia lent a hand, followed by Iran since at least 2005, according to the independent Nuclear Threat Initiative. The Scientific Research Council in Damascus appears to be directing the Syrian chemical weapons program, the NTI said. The US Congressional Research Service pointed to accessible information suggesting that the production and storage of nerve gas and mustard gas is concentrated in and around the cities of Al-Safira (southeast of Aleppo), Damascus, Hama, Homs and Latakia. Delivery vehicles include Scud ballistic missiles and launch systems, along with aerial bombs and shells, according to publicly-available information. However, \"there is not sufficient information in open sources to draw any conclusions about the security of Syria\'s CW arsenal,\" the NTI warned.