Militants fighting against the Syrian government have used chemical weapons against the army in Daraya near the capital, Damascus, military sources say.
According to a commander of the Syrian Presidential Guard, at least seven Syrian soldiers were killed on Saturday after they were attacked by a chemical weapon which produced a toxic yellow gas.
The soldiers were reportedly killed within an hour after inhaling the gas.
Foreign-backed militants have repeatedly threatened to use chemical weapons against the army and pro-government civilians in recent days.
They have also threatened to contaminate Syria's drinking water supply in a bid to kill all Alawite Shias and the supporters of President Bashar al-Assad.
The threat was made in a video posted on YouTube in which militants tested water contaminated with a lethal mixture on lab rabbits. The rabbits stopped breathing and their chests swelled shortly after drinking the poisoned water.
The militants had earlier released a footage in which lab rabbits were killed by inhaling poisonous gas.
The militants' use of chemical weapons come as the US and its allies have alleged that the Syrian government possesses the deadly weapons and is prepared to use them against militants.
Syria has been experiencing unrest since March 2011. Many people, including large numbers of army and security personnel, have been killed in the turmoil.
A recent UN report has revealed that militants from 29 countries have so far infiltrated into Syria to fight against the Damascus government, most of whom are extremist Salafists.
The Syrian government has repeatedly said that the chaos is being orchestrated from outside the country, and that a very large number of the militants operating in the country are foreign nationals.