Syrian military forces began destroying the remaining chemical weapons facilities in the country this weekend. Residents in Damascus and leaders in the Free Syrian Army, however, said that President Bashar Assad’s regime removed stockpiles of weapons from those facilities and hid them underground in at least five different locations near the capital.

Reuters reported Monday that Syria had begun the destruction of a dozen underground bunkers and hangars that were used for the production and storage of chemical weapons, but leaders of the Free Syrian Army (FSA), the largest opposition group fighting Assad’s forces, told International Business Times that Assad is hiding stockpiles at secret sites underground near Damascus. The rebel commander said Assad began removing stockpiles of chemical weapons in 2011 when the civil war first started. Since then, the regime has transferred weapons consistently to the secret sites.

According to FSA sources, the chemical weapon sites are located underground in Damascus near the Syrian air force headquarters; on the western side of Mount Qassioun, north of Damascus; on a main road that leads from Damascus to Aleppo; in the mountains next to the Adra military base; and at a facility in the al-Qaboun region inside a special forces facility.

Syria joined the Organization for the Prohibition of Chemical Weapons (OPCW) after it used sarin gas in an attack on the suburb of Ghouta Aug. 21, 2013. At least 1,000 people were killed in the attack, forcing international leaders to take action against the Syrian regime, which blamed the rebels for the attack. Before the attack in Ghouta, U.S. President Barack Obama had said his "red line," one that would prompt a military intervention in Syria, was the use of chemical weapons in Syria. The U.S. administration refrained from intervening in the conflict after Syria agreed to destroy its chemical stockpiles.

The OPCW has been overseeing the destruction of the country's chemical weapons since 2013. The monitoring group reported this weekend that the Syrian regime was in the process of destroying the last of its chemical weapons facilities. But those facilities have been vacated for months, FSA sources in Damascus told IBTimes, because the regime had transferred some of the weapons before the OPCW began its investigation.

It is not clear what kind of chemical weapons are being stored underground in the facilities. Since the attack in Ghouta in August of 2013, the Syrian regime has been accused of embedding chlorine in bombs that human rights groups said it dropped on civilians. The use of chlorine gas is prohibited under international law.

So far, the OPCW claims that Assad's chemical weapon facilities are being monitored and destroyed. Iraq claimed in September 2002 that it had no chemical weapons, but a New York Times report by C.J. Chivers in October 2014 revealed that American troops had found roughly 5,000 chemical warheads, shells or aviation bombs, while serving in Iraq.