Constructed with Chinese, Iranian, North Korean and possibly Russian assistance, the Aleppo Missile Factory is an underground facility that produces several variants of Scud and possibly Chinese M-9 missiles. [1] Missile production reportedly began in 1993. [2] Damascus also uses the site to perform chemical warhead fitting for its ballistic missile arsenal, and may have tested a Scud-B that was tipped with a chemical warhead in 2001. [3] In July 2007, an explosion at the site reportedly occurred during an attempt to weaponize a 500km Scud-C with a mustard gas warhead. [4] The explosion killed 15 Syrian military personnel, dozens of Iranian missile engineers, and may have released VX and Sarin nerve agents into the immediate area. [5]

Sources:
[1] Angelo M. Codevilla, "Missiles, Defense and Israel," The Threat of Ballistic Missiles in the Middle East, ed. Arieh Stav (Portland, OR: Sussex Academic Press, 2004), p.61; Anthony H. Cordesman, Arab-Israeli Military Forces in an Era of Asymmetric Wars (Westport, CT: Praeger Security International, 2006), p.363.
[2] Greg W. Gerardi and James A. Plotts, "An Annotated Chronology of DPRK Missile Trade and Developments," The Nonproliferation Review, Fall 1994, p.79.
[3] Anthony H. Cordesman, "Syrian Weapons of Mass Destruction," 1st Working Draft, 2 June 2008, p.14, www.csis.org.
[4] Robin Hughes, "Explosion Aborts CW Project Run by Iran and Syria," Jane's Defence Weekly, 17 September 2007, www.janes.com.
[5] Robin Hughes, "Explosion Aborts CW Project Run by Iran and Syria," Jane's Defence Weekly, 17 September 2007, www.janes.com.