The deadly nerve agent sarin has possibly been used in Syria, the United Nations' acting disarmament chief has told the UN Security Council.

Kim Won-soo spoke to reporters on Tuesday after briefing the Security Council behind closed doors on the latest report from the Organisation for the Prohibition of Chemical Weapons (OPCW).

The report said OPCW investigators - who looked at 11 allegations of chemical weapons use - came across one instance of blood samples indicating "that individuals were at some point exposed to sarin or a sarin-like substance".

"Further investigation would be necessary to determine when or under what circumstances such exposure might have occurred," the report said.

The OPCW only carries out fact-finding missions, but the Security Council in August established an expert team that will seek to assign blame for chemical attacks in Syria's war.

Malik Ellahi, an OPCW spokesman, said there hasn't yet been a decision taken on which body should conduct further investigation into the blood samples.

The opposition Syrian National Coalition has urged the OPCW to investigate an alleged chemical attack last month in the Damascus suburb of Moadamiyeh - an area besieged by government forces.

The opposition blamed President Bashar al-Assad's regime, but the Syrian government denied involvement and blamed rebels and the Islamic State of Iraq and the Levant.

Doubts expressed

Kim said OPCW fact-finding teams were still assessing reports of alleged chemical attacks, carrying out investigations, and sending their findings to the Security Council expert body, which is known as the Joint Investigative Mechanism (JIM).

"Then, the JIM will do its own investigation," he said.

The report raises other outstanding issues, including questions about the Syrian government's completion of the destruction of its declared chemical weapons stockpile as called for under an international agreement.

That agreement came after an infamous sarin attack on the Damascus suburb of Eastern Ghouta in August 2013 that killed more than 1,000 people and drew international horror.

Syria agreed the following month to destroy its entire supply of chemical weapons under a deal negotiated with the US and Russia.

The last batch of 1,300 tonnes of chemicals declared to the OPCW was handed over in June 2014, but several Western governments have expressed doubts that the Assad regime declared its full arsenal.