While chemical agents allegedly used by ISIS so far have been among its least effective weapons, the psychological impact on civilians is considerable.
There are "extremely worrying" signs that the ISIS group may be making its own chemical arms and have used them already in Iraq and Syria, a global watchdog said Tuesday.
Organisation for the Prohibition of Chemical Weapons head Ahmed Uzumcu said his body's fact-finding teams have found evidence of the use of sulphur mustard in attacks in the two war-torn countries.
"Although they could not attribute this to Daesh... there are strong suspicions that they may have used it (chemical weapons)," Uzumcu told AFP, using the alternative name for the jihadist group.
"Secondly the suspicions are that they may have produced it themselves, which is extremely worrying," Uzumcu said on the sidelines of a three-day conference at the OPCW's Hague-based headquarters.
"It proves that they have the technology, know-how and also access to the materials which might be used for the production of chemical weapons," Uzumcu said.
CIA director John Brennan in February told CBS News that ISIS fighters had the capability to make small quantities of chlorine and mustard gas.
Uzumcu did not point to any specific attacks, but last month ISIS mounted a deadly gas attack against Syrian troops at a government-controlled airbase outside the divided eastern city of Deir Ezzor, according to the SANA state news agency.
The attack was the latest in a string of suspected mustard gas attacks by the jihadists in Syria and neighbouring Iraq.
On March 9, a suspected ISIS gas attack on the Iraqi town of Taza, south of Kirkuk, killed three children and wounded some 1,500 people, with injuries ranging from burns to rashes and respiratory problems.
While chemical agents allegedly used by ISIS so far have been among its least effective weapons, the psychological impact on civilians is considerable.
A total of 25,000 people fled their homes in and around Taza last month, fearing another attack.
Uzumcu also urged vigilance by other nations to guard against any chemical attacks outside Syria and Iraq.
Russia last month pushed for measures at the United Nations to monitor extremist groups fighting in Syria, warning of a "clear and present threat" that they could stage chemical attacks, possibly in Europe.