On April 4, a chemical weapons incident in Syria's Idlib province claimed the lives of some 80 people and inflicted harm on an additional 200 civilians.
Damascus, along with its Russian and Iranian allies, has said that the April 4 casualties were caused when a conventional air strike on an al-Qaeda weapons depot nearby caused an explosion, releasing the deadly gases.
But Moscow, the closest ally of Syrian President Bashar Assad, is leading efforts to sideline the OPCW's existing fact-finding mission by calling for a new "full-scale and thorough investigation".
U.S. Secretary of Defense Jim Mattis initially said 20 percent of Syria's air force had been destroyed in the cruise missile attack, but later backtracked and said that 20 aircraft had been rendered inoperable.
The team "deserves our full confidence", the Belgian representative to the OPCW told the meeting on Wednesday.
Defence ministry spokesman Igor Konashenkov issued a statement on Thursday questioning how the samples were collected and how the analysis could have been done so quickly. Where, by whom and how were the samples taken?
"If it is true that sarin was used in Khan Shaykhun, how can the OPCW then account for the fact the charlatans from the White Helmets organization were hustling and bustling inside sarin clouds with no protective gear on?"
The draft decision proposed by Russian Federation and Iran also sought to urge member states to "provide national experts for participation in the investigation".
That would have enabled Moscow to deploy its own experts alongside the OPCW's independent teams in a bid "to discredit the results" so far, one source close to the discussions said.
Assad insists his regime handed over all chemical weapons stockpiles in 2013, under a deal brokered by the regime's key ally, Russia, to avoid threatened USA military action.
It came after a joint UN-OPCW investigation concluded in October that the Syrian air force had dropped chlorine barrel-bombs from helicopters on three opposition-held villages in 2014 and 2015.
The attack led to widespread global outrage, including attempts to pass a UN Security Council resolution condemning it, blocked last week by Russian Federation.
"Actually, since the first attack a few years ago that happened in Aleppo by the terrorists against our army, we asked the United Nations to send investigation delegation in order to prove what we said about the terrorists having gases used against our army, and later many incidents happened in that way, and they didn't send any delegation. Also at the time some of the sites where they were stored [were] besieged...and the OPCW just couldn't get to them to verify them", he says.