"The Assad regime is responsible for innumerable atrocities some of which rise to the level of war crimes and crimes against humanity," Pompeo said. "Today I am announcing that the United States has concluded that the Assad regime used chlorine as a chemical weapon on May 19."
In May, the US received multiple reports that appeared to indicate chemical exposure after an attack by Assad's forces in northwest Syria but had not made any definitive conclusion as to whether chemical weapons were used.
Syria denied that it launched a chemical attack at the time, but a UN panel has attributed over two dozen chemical attacks to the Assad regime over the course of the country's civil war, according to NPR.
The United States has bombed Syria twice over suspected use of chemical weapons by Assad: in April 2017 and April 2018.
In 2012, former president Barack Obama stated that Syria's use of chemical weapons would be a red line. He never conducted a military strike in response after it was confirmed that Syria had conducted a chemical weapons attack, but he did work with Russia to remove chemical weapons from Syria.
In 2014, former secretary of state John Kerry told NBC's Meet the Press that the US, together with Russia had removed "100% of the declared chemical weapons from Syria." Chlorine was not one of the chemical agents removed since it's commonly used for water treatment and other industrial purposes, according to NPR.
"This is different in some sense because it was chlorine... but know that President Trump has been pretty vigorous in protecting the world from the use of chemical weapons," said Pompeo. He declined to say what the US's response to the attack would be.
Pompeo added that new sanctions had been placed on two Russian entities for providing fuel to the Syrian government.