(CNN) The United States has concluded that the Assad regime carried out a chlorine attack on Syria in May, Secretary of State Mike Pompeo announced Thursday.

"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," Pompeo said at a news conference in New York. "It is also the latest instance in a long pattern of Assad's chemical weapons attacks that have killed or wounded thousands of Syrians."

The official assessment came months after the State Department's initial warning about an alleged chemical attack on May 19.

Pompeo said Wednesday that the US "will not allow these attacks to go unchallenged, nor will we tolerate those who choose to conceal these atrocities." He also announced an additional $4.5 million contribution by the US to the Organization for the Prohibition of Chemical Weapons "to help its investigations of continued chemical weapons use in Syria."

The Trump administration ordered retaliatory strikes against Syrian regime targets in 2017 and 2018 after reports of chemical weapons attacks by Syrian President Bashar al Assad's forces.

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