The United States warned Monday that efforts to uphold the global ban on the production and use of chemical weapons are at a “critical juncture” in the wake of their use during Syria’s civil war.

As the Syrian government and its allies seek to discredit conclusions by international investigators that Damascus has repeatedly used such weapons, Andrea Hall of the U.S. National Security Council said the ban on chemical weapons, such as nerve agents and poison gas, is now under threat.

“We must take every opportunity to deter states from using chemical weapons,” Hall said. “If we fail to take action now, non-state actors use will also rise.”

Hall was speaking on the opening day of the annual conference of state parties to the Organization for the Prohibition of Chemical Weapons, which won the 2013 Nobel Peace Prize for overseeing the global ban on weapons, such as nerve agents and poison gas.

“Chemical weapons use by the Syrian Arab Republic remains the most serious violation of the CWC in the convention’s 20-year history and the greatest modern challenge to the global norm against chemical weapons use,” Hall told delegates in The Hague.

A joint investigation team made up of OPCW and UN experts has concluded that the Syrian government used chlorine gas in at least two attacks in 2014 and 2015, and used the nerve agent sarin in an aerial attack on Khan Sheikhoun last April 4 that killed about 100 people and affected about 200 others.

The team also accused the Islamic State extremist group of using mustard gas in 2015 and again in September 2016 in Um Hosh in Aleppo.

Russia, Syria’s staunch ally, recently vetoed a United Nations Security Council resolution that would have extended the mandate of the UN-OPCW Joint Investigative Mission. That mandate expired earlier this month,

Syrian Deputy Foreign Minister Faisal Mekdad branded the investigation unprofessional and biased.

“So we are always seeing politicized reports, false conclusions aiming to bring even more pressure to bear on Syria,” he said.

British Ambassador Peter Wilson echoed U.S. concerns, saying that the chemical weapons convention “is under unprecedented attack.”

Hall also rejected Syria’s criticism of the UN-OPCW investigation team, saying that the team had produced, “serious, thorough, technical and compelling reports.”

(AP)