Opposition forces in Syria on Monday accused government forces of using chemical weapons several times so far during the country’s ongoing civil war.

Speaking to the London-based a-Sharq al-Awsat newspaper, Free Syrian Army official Louay Almokdad ‎emphasized that the alleged use of white gas against rebel forces in Homs on Sunday was not the first time President Bashar Assad’s forces unleashed such an attack.

Six people were reported killed and dozens more injured in that attack, which hasn’t been confirmed by any outside body.

Almokdad said that the Syrian opposition has “recorded use of chemical weapons 18 times in Damascus, Idlib, Homs and Hama.” However, he did not provide solid evidence to substantiate the claim.

Qatar-based satellite channel Al Jazeera reported that 10 people suffered serious injuries from the chemical attack in Homs, including blindness and paralysis, and over 50 more suffered other injuries.

Opposition activists posted a video online that they claimed shows a victim of the alleged chemical attack suffering from breathing difficulties.

There have been conflicting reports as to what kind of chemical was allegedly used in the attack. Syria has large supplies of the sarin nerve agent as well as VX and mustard gas. According to one report from the Syrian American Medical Society, the chemical used was Agent-15, also known as  3-Quinuclidinyl Benzilate, a less deadly but still dangerous weapon.

The international community has raised concerns that Syria’s large chemical weapons store could be used against the rebels, or fall in opposition hands. Reports have ramped up over recent weeks that the embattled Assad was preparing to use chemicals as the country’s 21-month-old civil war rages on.

The US has called the use of chemical weapons a “red line” that may invite international intervention. On Saturday, Russian Foreign Minister Sergei Lavrov said the Syrian government had moved the chemical weapons from many arsenals to just “one or two centers” to properly safeguard them.

On Monday, UN envoy Lakhdar Brahimi emerged from a meeting with Assad saying that the Syrian situation was “worrying.” He said no progress was being made toward a negotiated solution.