Pompeo said Assad's regime was responsible for a chlorine attack on Syria's northwest province of Latakia on May 19.
"This attack was part of the Assad regime's ongoing violent campaign in Idlib, which has killed more than 1,000 innocent Syrians and displaced hundreds of thousands more," he said. "It is also the latest instance in a long pattern of Assad's chemical weapons attacks that have killed or wounded thousands of Syrians."
Days after the attack, the State Department warned Assad's government that its military operations were being closely watched.
Jim Jeffrey, U.S. special representative for Syria engagement, said during a special briefing on Syria Thursday that a number of civilians were injured while no one was killed in that May attack, which was an attempt by Assad to capture the area from opposition forces.
President Donald Trump views the attack as a "reckless escalation" because it shows Assad is willing to resort to chemical weapons to compensate for his lack of infantry, he said.
"One of the many reasons for that is that we fear that the regime, which has very weak infantry forces, will try to use chemical weapons once again to make up for its inability to seize ground by combat," he said.
In connection, the U.S. Treasury said it sanctioned one company, three people and five vessels, all Russian, for delivering jet fuel to Russian military forces operating in Syria in support of Assad's "bombing campaigns that destroyed numerous hospitals, schools and public spaces, resulting in civilian deaths."
"Assad's despotic regime is under an international spotlight for using chemical weapons and committing atrocities against innocent Syrian civilians and they rely on these types of illicit networks to stay in power," Undersecretary for Terrorism and Financial Intelligence Sigal Mandelker said in a statement. "The U.S. is determined to cut off access to the international financial system from those who conspire to violate our sanctions, including those who enable the brutal war machine in Syria."
Pompeo announced Thursday the State Department will provide an additional $4.5 million to the Organization for the Prohibition of Chemical Weapons.
"The United States will not allow these attacks to go unchallenged, nor will we tolerate those who choose to conceal these atrocities," he said.
Thomas DiNanno of the U.S. Bureau of Arms Control, Verification and Compliance said the money goes toward the OPCW's mandate of investigating the use of chemical weapons in Syria.
"This work is vital," he said. "It is important to ensure accountability."