The 15-nation council was due to vote on Thursday on rival USA and Russian bids to renew the global inquiry, diplomats said on Wednesday, a move that could trigger Russia's 10th veto to block action on Syria.
Diplomats say there is little support among the 15-member council for the Russian draft, which Russian UN Ambassador Vassily Nebenzia has said aims to correct "systemic errors" of the inquiry, known as the Joint Investigative Mechanism (JIM).
The United Nations Security Council is due to vote on Thursday on rival USA and Russian bids to renew an worldwide inquiry into chemical weapons attacks in Syria, diplomats said, a move that could trigger Russia's 10th veto to block action on Syria.
Russian Federation and Iran back President Bashar al-Assad in the conflict, while Turkey and the United States support differing rebel groups fighting the government.
The United States, Britain and France have insisted that the JIM should be allowed to continue its work and that dozens of other cases of chemical weapons use in Syria must be investigated.
"How then can we trust Russia's support for supposed peace in Syria?" she said. Diplomats said Russian Federation was expected to also request a vote.
UNSC to vote on Syria chem gas attacks probe
In April, more than 80 people were killed in a sarin attack on the rebel-held Syrian town of Khan Sheikhoun - an attack that prompted the United States to launch 59 Tomahawk cruise missiles at a Syrian airbase.
Russian Ambassador Vassily Nebenzia said he opposed the USA measure because it was "geared toward entrenching the inherent flaws of the JIM" and did nothing to improve its work.
Haley tweeted after the vote that "by using the veto to kill a mechanism in Syria that holds users of chemical weapons accountable, Russian Federation proves they can not be trusted or credible as we work towards a political solution in Syria".
"Let there be no doubt: we have unleashed a monster here", said Delattre.
The mandate for the joint inquiry by the United Nations and the Organization for the Prohibition of Chemical Weapons (OPCW), which found the Syrian government used the banned nerve agent sarin in an April 4 attack, expires at midnight Thursday. That averted a US military strike in response to an alleged chemical weapons attack that killed hundreds in the Damascus suburb of Ghouta.
Previous reports by the JIM have found that Syrian government forces were responsible for chlorine attacks on three villages in 2014 and 2015, and that the Islamic State group used mustard gas in 2015.