Secretary of State Mike Pompeo, speaking on the sidelines of the U.N. General Assembly on Thursday, said the United States has concluded the Syrian government used chemical weapons in an attack earlier this year.

Pompeo said U.S. officials had determined Syrian President Bashar Assad's government had used chlorine in the attack on opposition forces in Idlib province on May 19.

“The Assad regime is responsible for innumerable atrocities some of which rise to the level of war crimes and crimes against humanity,” Pompeo told a news conference.

U.S. officials in May said they’d received reports consistent with a chemical attack in northwest Syria but did not offer a definitive conclusion at the time.

In April 2018, the U.S., Britain and France launched airstrikes in Syria in retaliation for what they described as a suspected chemical attack that killed more than 40 people earlier that month.

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“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,” Pompeo said.

He added that the U.S. would provide an additional $4.5 million to the Organization for the Prohibition of Chemical Weapons to fund investigations into other instances of suspected Syrian chemical-weapons use.

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Pompeo also called on the Assad government to release thousands of unjustly detained prisoners, including American journalist Austin Tice, who has been missing and presumed held by the government for seven years.

"He is just one of a number of American citizens held in Syria and we call upon the Syrian regime to release them all."

The Associated Press contributed to this report.