THE HAGUE, Netherlands — The global chemical weapons watchdog said Friday that its investigators found “reasonable grounds to believe” Syria’s air force dropped two cylinders containing chlorine gas on the city of Douma in April 2018, killing 43 people.

A report by a team from the Organization for the Prohibition of Chemical Weapons offered the latest confirmation that the regime of the Syrian dictator, President Bashar Al-Assad, used chemical weapons during his country’s grinding civil war.

“The use of chemical weapons in Douma — and anywhere — is unacceptable and a breach of international law,” the organization’s director-general, Fernando Arias, said.

Syrian officials did not immediately respond to a request for comment on the findings. Syria joined the Organization for the Prohibition of Chemical Weapons in 2013 under pressure from the international community after being blamed for another deadly chemical weapon attack, but does not recognize the investigation team’s authority and has repeatedly denied using chemical weapons.

Bringing perpetrators in Syria to justice remains a long way off. Syria’s ally Russia has, in the past, blocked efforts by the UN Security Council to order an International Criminal Court investigation in Syria.

“The world now knows the facts. It is up to the international community to take action, at the OPCW and beyond,” Mr. Arias, a veteran Spanish diplomat, said.

The Organization for the Prohibition of Chemical Weapons team previously identified Syrian forces as responsible for three chemical attacks in Latamneh in March 2017 and one in Saraqeb in February 2018.

The organization said “reasonable grounds to believe” is the standard of proof consistently adopted by international fact-finding bodies and commissions of inquiry that investigate potential violations of international law.

Friday’s report said that standard was met through evidence indicating at least one Syrian air force Mi8/17 helicopter dropped two yellow cylinders on Douma during a government military offensive to recapture the city,.

One of the cylinders hit the roof of a three-story residential building and ruptured, “rapidly released toxic gas, chlorine, in very high concentrations, which rapidly dispersed within the building killing 43 named individuals and affecting dozens more,” according to the report.

A second cylinder burst through the roof of another building into an apartment below and only partially ruptured, “mildly affecting those who first arrived at the scene,” the report added.

Syrian authorities refused the investigation team access to the sites of the chlorine attacks. The country had its OPCW voting rights suspended in 2021 as punishment for the repeated use of toxic gas, the first such sanction imposed on a member nation.

The investigation by the organization’s team, which was set up to identify perpetrators of chemical weapon attacks in Syria, built on earlier findings by an Organization for the Prohibition of Chemical Weapons fact-finding mission that chlorine was used as a weapon in Douma.

The investigators interviewed dozens of witnesses and studied the blood and urine of survivors as well as samples of soil and building materials, according to the watchdog agency.

As part of the probe, they also assessed and rejected alternative theories for what happened, including Syria’s claim that the attack was staged and that bodies of people killed elsewhere in Syria were taken to Douma to look like victims of a gas attack.

The report found that the two cylinders carrying chlorine were modified and filled at the Dumayr air base and the helicopter or helicopters that dropped them were under control of the Syrian military’s elite Tiger Force.

The Organization for the Prohibition of Chemical Weapons team “considered a range of possible scenarios and tested their validity against the evidence they gathered and analysed to reach their conclusion: that the Syrian Arab Air Forces are the perpetrators of this attack,” the organization said in a statement.

British diplomats at the Organization for the Prohibition of Chemical Weapons tweeted that they were studying the report, adding: “We will work with partners on next steps. #NoImpunity”

Survivors reached by the Associated Press in the aftermath of the Douma attack said they were overwhelmed by the smell of chlorine. Activists said many of the dead were found with foam around their mouths, an indicator of suffocation. Medical workers said they treated people for symptoms that included difficulty breathing and fainting.

The United States, Britain and France blamed Syrian government forces and launched punitive airstrikes. Syria denied responsibility.

Douma was the final target of the government’s sweeping campaign to seize back control of the eastern Ghouta suburbs of Damascus from rebels after seven years of revolt. Militants gave up the town days after the alleged attack.

The ongoing conflict that started in Syria more than a decade ago has killed hundreds of thousands and displaced half the country’s prewar population of 23 million.