Syrian President Bashar Assad’s forces used chemical weapons during an assault against the final rebel-held region of the country, according to Secretary of State Mike Pompeo.

“The United States has concluded that the Assad regime used chlorine as a chemical weapon on May 19th,” Pompeo told reporters at the United Nations General Assembly in New York. “The United States will not allow these attacks to go unchallenged, nor will we tolerate those who choose to conceal these atrocities.”