A fact-finding mission of the global anti-chemical weapons watchdog has found indications that some people in Syria were exposed to deadly sarin gas, or a compound like it, according to a report the United Nations released on Monday.

The findings come in the latest monthly report on Syria from the Organization for the Prohibition of Chemical Weapons (OPCW) chief Ahmet Uzumcu. U.N. Secretary-General Ban Ki-moon attached it in a Dec. 29 letter to the 15-nation Security Council.

Uzumcu's report said his agency's fact-finding mission in Syria was looking into charges by the Syrian government that chemical weapons were used in 11 instances. The report did not specify when the alleged 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 government has long accused opposition fighters, who have been seeking for nearly five years to oust the country's president, of using chemical weapons. Western-backed rebels in Syria have repeatedly denied using chemical weapons.

Western officials say it is unlikely rebels would have the capability to deploy sarin gas.

Uzumcu said the source of the sarin or sarin-like compound was unclear, adding that the OPCW fact-finding mission "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 entire chemical weapons program under a deal negotiated with the United States and Russia after hundreds of people were killed in a sarin gas attack in the outskirts of the capital, Damascus.

At the time, Washington was threatening the government of Syrian President Bashar al-Assad with air strikes.

The OPCW had previously determined that mustard gas was used in a Syrian town where Islamic State insurgents were battling another group.

The last of 1,300 tons of chemical weapons declared to the OPCW was handed over in June 2014, but several Western governments have expressed doubt that Assad's government declared its entire arsenal.

The OPCW has reported previously that chlorine has also been used illegally in systematic attacks against civilians in Syria.

Several international investigations have determined that chemical weapons have been used in Syria, though none has so far assigned blame. A U.N.-OPCW joint investigative mission has been given the task of determining who was behind those attacks.