An international chemical weapons watchdog on Monday formally accused Syria of using poison gas against opposition forces in 2018, in an attack that occurred a day after militants shot down a Russian warplane in the same area.

The report by the Organization for the Prohibition of Chemical Weapons is the second in the past year to specifically identify Syrian military units that allegedly used the banned weapons in fighting rebel groups in the country’s civil war. The February 2018 incident took place five years after Syria publicly renounced the use of chemical weapons and nearly a year after President Donald Trump launched a missile strike against Damascus over its use of deadly nerve agents.

The OPCW has launched a systematic review of the evidence in dozens of alleged chemical weapons attacks in Syria as it seeks to pinpoint blame. Both Syria and Russia — a staunch defender of Syrian President Bashar al-Assad — have denied any use of chemical weapons by Damascus and have instead accused the rebels of staging attacks and fabricating evidence.

The OPCW’s 41-nation executive council is expected to seek penalties for Syria at a meeting in The Hague this month, including a possible referral to the U.N. Security Council.

OPCW experts reviewed 400 gigabytes of data and interviewed dozens of witnesses in its investigation of the Feb. 4, 2018, attack in Saraqeb, then a rebel-held town in northern Syria and the target of a government offensive. After nightfall, a helicopter from Syria’s elite Tiger Forces dropped a pair of makeshift bombs on the town, including at least one that contained chlorine, an industrial compound used by Syrian forces as a chemical weapon, the report said.

“During ongoing attacks against Saraqeb, a military helicopter of the Syrian Arab Air Force under the control of the Tiger Forces hit eastern Saraqeb by dropping at least one cylinder,” the report said. “The cylinder ruptured and released a toxic gas, chlorine.” At least 12 people were injured.

Syrian officials alleged that the rebel groups staged the attack, perhaps by dropping a chlorine canister from a radio tower — which OPCW experts noted was more than a mile away.

The attack occurred a day after the downing of a Russian Su-25 warplane that crashed less than 10 miles from Saraqeb, killing the pilot. The report, citing military experts, suggested that the chemical attack might have been intended as “punishment” for the attack against Syria’s Russian ally.

Russia, which co-sponsored a 2013 agreement with the United States to destroy Syria’s chemical weapons stockpile, has been accused by U.S. officials of covering up — or even abetting — Syria’s use of chemical weapons in the years since, while defending Assad’s behavior at the U.N. and in other forums.

“Accusations that the Syrian government continues to use chemical weapons against its own people were considered anathema in Moscow,” said Hanna Notte, a researcher at the James Martin Center for Nonproliferation Studies. a Monterey, Calif., nonprofit organization. “If proven true, they would have embarrassed Russia’s role in the original chemical disarmament effort, while also undermining the path toward Syria’s normalization.”