US Secretary of State Rex Tillerson, British Foreign Secretary Boris Johnson and French Foreign Minister Jean-Yves Le Drian also said it's "essential" to seek justice for the victims and hold those responsible accountable.
Their joint statement follows the latest report by experts from the Joint Investigative Mechanism, or JIM, blaming the Syrian government for an attack using sarin nerve gas that killed about 100 people in Khan Sheikhoun in April, and the Islamic State extremist group for a mustard gas attack at Um Hosh in Aleppo in September 2016.
Russia vetoed a US-sponsored council resolution on October 24 that would have renewed the mandate of the experts from the UN and the Organization for the Prohibition of Chemical Weapons which expires on November 14. Russia's ambassador said Moscow wanted to see the JIM report, which was issued two days later.
The Western powers praised the JIM report. "We have full trust in the JIM's findings, its professionalism and independence," the top US, British and French diplomats said Wednesday.
But Russia, Syria's most important ally, strongly criticized the JIM's conclusion blaming President Bashar Assad's government for the Khan Sheikhoun attack on April 4. Russian officials denounced the experts' investigative methods and failure to visit the town.
At a Security Council meeting Tuesday to consider the JIM report, Russia's deputy UN ambassador Vladimir Safrankov complained about the hunt for "mythical or invented chemical weapons in Damascus."
In September 2013, Syria accepted a Russian proposal to relinquish its chemical weapons stockpile and join the Chemical Weapons Convention. That averted a US military strike in response to an alleged chemical weapons attack that killed hundreds in the Damascus suburb of Ghouta.
Syria's UN Ambassador Bashar Ja'afari on Tuesday called the JIM report fabricated and said the government "considers the use of chemical weapons an immoral act that must be condemned."
Russia and the United States have submitted rival resolutions to extend the JIM's mandate but it's unclear whether a compromise can be reached by November 16. If the mandate isn't extended, the JIM will cease operations immediately.
US Ambassador Nikki Haley told the Security Council Tuesday that the US revised its resolution to include some points in the Russian draft.
But she claimed the Russian draft would keep Syria from being investigated and said Russia is continuing "to push unacceptable language only meant to undermine the investigators and divide this council."
Tillerson, Johnson and Le Drian stressed that "there is more work for the JIM to do" and "it is vital for the international community to continue to investigate cases where chemical weapons have been used in Syria."
They pointed to a recent OPCW report that a sarin attack that injured 50 people "most likely" took place days before the Khan Sheikhoun attack in the town of Ltamenah, about 15 kilometers away.
"We therefore urge the United Nations Security Council to maintain the JIM's investigative capacity," they said.
The US, UK and French diplomats also called on the OPCW's executive council to respond to the JIM report and "send an unequivocal signal that those responsible for the use of chemical weapons will be held responsible."