Syrian government forces carried out a chlorine attack in May, the first confirmed violation of the international accord banning chemical weapons since President Trump authorized a U.S. military strike on Syria in 2018 over its alleged use of poison gas, a new U.S. intelligence assessment says.

The episode took place on May 19 near the village of Kabana as President Bashar al-Assad’s forces sought to subdue resistance in Latakia province, a senior U.S. official said.

Secretary...