An international watchdog on Thursday set a 90-day deadline for Syria to come clean about chemical weapons it kept hidden after claiming it had surrendered its entire stockpile to inspectors in 2014.

The Organization for the Prohibition of Chemical Weapons formally accused Syria’s government of deceiving investigators and violating its commitments by keeping a secret reserve of the deadly nerve agent sarin, which it used in multiple attacks in the spring of 2017.

Syria has long been accused of cheating on the landmark 2013 accord in which it agreed to eliminate its chemical arsenal, but Thursday’s decision was the first attempt by the international watchdog to seek to compel President Bashar al-Assad to acknowledge the violations and account for the hidden stash.

The vote by the OPCW’s 41-nation executive council in The Hague comes three months after the organization’s investigators released findings that firmly linked Damascus to multiple sarin-gas attacks on rebel-held villages in early 2017. Both Syria and its ally Russia blame rebel militias for the attacks.

The council gave Syria 90 days to explain where it kept the hidden sarin and how much, if any, it still possesses. A failure to provide information would eventually lead to a referral to the U.N. Security Council for possible economic sanctions. The resolution, which passed by a vote of 29 to 3 with nine countries abstaining, also called for accountability for Syrian officials who authorized the use of sarin.

“Good day for international security and [for] no impunity,” Luis Vassy, France’s ambassador to the Netherlands, said in a Twitter posting after the vote was tallied.

Until 2013, Syria possessed one of the world’s largest arsenals of chemical weapons. After a sarin attack on a Damascus suburb killed 1,400 civilians that year, Assad — facing a possible airstrike by the United States and France — agreed to turn over his entire stockpile for destruction under international supervision. Some 1,300 tons of deadly chemicals were destroyed by September 2014.

U.S. intelligence officials suspected at the time that Assad concealed some of the weapons, a suspicion that was borne out when the Syrian army resumed using sarin in attacks on rebel-held towns in 2017. The Trump administration ordered a missile strike on a Syrian air base in April 2017 to punish Assad for the use of nerve gas. The OPCW’s Investigation and Identification Team forensically linked the Syrian government to the sarin attacks three years later.

Western governments and human rights groups praised the OPCW’s decision, despite widespread skepticism that Syria would comply with the demands.

“Ultimately, this is a regime that has acted with impunity every single day since the spring of 2011” when the country’s civil war began, said Charles Lister, director of the Syria program at the Middle East Institute, a Washington nonprofit. “Though an OPCW declaration might make us feel good, it won’t stop the Assad regime from continuing to flout every law in the book in pursuit of its survival.”