The global chemical watchdog said Monday there were "no reasonable grounds" to conclude that a 2017 "attack" in Syria blamed on the IS group contained chemical weapons.
The Organisation for the Prohibition of Chemical Weapons said Damascus in November 2017 reported "use of toxic chemicals in an attack by the terrorist organisation ISIS against another terrorist group called Aknaf Beit Almaqdis."
The alleged attack took place at the sprawling Yarmouk district in Damascus in October that year.
It resulted in several cases of breathing difficulties "and loss of consciousness in the ranks of Aknaf terrorist group," Damascus told the OPCW.
But the Hague-based body said after investigating, its Fact-Finding Mission (FFM) concluded that "there are no reasonable grounds to determine that toxic chemicals were used as a weapon in the reported incident."
Set up in 2014 the FFM investigates the use of chemical weapons in Syria, but it cannot identify the perpetrators behind the attacks.
The OPCW's investigators based their findings on chemical sample analyses, interviews with witnesses, video and photo evidence and documents and correspondence with the Syrian government.
"The samples analysis results provided no indication that chemicals were used as a weapon," the OPCW said in a statement.
"There was no detection of the presence of scheduled chemicals, their precursors and, or their degradation products, nor of riot control agents, chlorinated organic chemicals or compounds containing chemically reactive chlorine," it said.
The FFM also tried to interview witnesses who were present "in areas of interest at the time of the reported incident."
This was unsuccessful because several witnesses had died, or were missing, while others who had initially agreed to provide testimony "ultimately declined to provide their account of the events to the FFM," the OPCW said.
The war in Syria has killed more than half a million people since it erupted in March 2011.
Syria agreed in 2013 to join the OPCW, shortly after an alleged chemical gas attack killed more than 1,400 people near Damascus.
But the global watchdog had since accused President Bashar al-Assad's regime of continuing to attack civilians with chemical weapons in the Middle East country's brutal civil war.
Damascus denies the charges.