The United States has concluded that Syria used chlorine gas in an attack against rebels last May, saying Thursday that it was the latest use of chemical weapons by President Bashar al-Assad’s government in the eight-year civil war but stopping short of threatening a military response.

Secretary of State Mike Pompeo warned Mr. Assad’s government that “we’re going to do everything we can reasonably do to prevent this kind of thing from happening again.”

But he said that chlorine attacks amounted to a “different situation” than the suspected use of sarin, a nerve agent, that killed 80 people and provoked missile strikes against a Syrian air base by the Trump administration in April 2017.

One year later, in April 2018, at least 40 people died in a chemical attack that may have involved sarin or chlorine — or possibly elements of both. That galvanized the United States, Britain and France to launch airstrikes against Syrian chemical weapons storage facilities and military depots.