The UN's Organisation for the Prohibition of Chemical Weapons (OPCW) said in its report on Monday that some people have been exposed to sarin gas in Syria.
The OPCW chief Ahmet Uzumcu said in a report that was attached by UN Secretary-General Ban Ki-moon to a Dec.29 letter to the 15-nation Security Council, that chemical weapon usage by the Syrian regime was found in 11 instances.
However, the report did not give any specific information about the time of when the toxic gas attacks occurred.
"In one instance, analysis of some blood samples indicates that individuals were at some point exposed to sarin or a sarin-like substance," Uzumcu said. "Further investigation would be necessary to determine when or under what circumstances such exposure might have occurred."
The Syrian regime denied allegations of using chemical weapons many times. Also Western officials say that opposition fighters have no capability of deploying sarin gas.
Uzumcu said the source of the sarin or sarin-like compound was unclear, adding that the OPCW "did not come across evidence that would shed more light on the specific nature or source of the exposure."
Syria agreed in September 2013 to destroy its chemical weapons stockpile under a deal negotiated between Washington and Moscow. This deal constitutes the resolution numbered 2118 that was imposed by the UN Security Council, which had a deadline of 30 June 2014 for the destruction of the chemical weapons.
In August, the UN Secretary General Ban Ki-moon started a new international inquiry into Syrian chemical weapon attacks during the civil war in the country.
It is not believed that Syria destroyed all of its toxic chemical weapons and facilities due to the incident last month in where forces loyal to Syrian regime leader Bashar al Assad reportedly carried out a chemical attack on the Western Ghouta suburb which is in the outskirts of the capital Damascus, killing 10 people.
The OPCW also previously reported that mustard gas and chlorine gas were used in a Syrian town in where DAESH terrorists were fighting.
The OPCW said that Syria was given 1,300 tonnes of chemical weapons in June 2014.
In September, the UN Security Council announced it would launch an investigation into the 2013 incident after member state Russia lifted its objections to the probe.
UN estimates suggest that at least 250,000 people have been killed since the war began in Syria in March 2011, and that half of the country’s population has been displaced both internally and externally.