An investigation by the international chemical weapons watchdog has confirmed Friday that sarin nerve gas was used in a deadly April 4 attack on a Syrian town.

The Organization for the Prohibition of Chemical Weapons said the attack on the town Khan Sheikhoun, in Syria's Idlib province, left more than 90 people dead, including women and children.

OPCW Director-General Ahmet Uzumcu has condemned the attack as an "atrocity" and says the "perpetrators of this horrific attack must be held accountable for their crimes."

The investigation did not apportion blame. Its findings will be used by a joint United Nations-OPCW investigation team to assess who was responsible.

President Trump cited the images when he launched a punitive strike days later, firing cruise missiles on a Syrian government-controlled air base from where U.S. officials said the Syrian military had launched the chemical attack.

It was the first direct American assault on the Syrian government and Trump's most dramatic military order since becoming president months before.

Syrian President Bashar Assad denied responsibility for the attack. Syrian ally Russia argued after the incident that the victims had died of exposure to toxic agents released when Syrian warplanes hit a rebels' chemical weapons depot.

The conclusion that sarin was used had been expected. The OPCW Director-General, Ahmet Uzumcu, said two weeks after the attack that tests carried out on samples taken from victims and survivors indicated they had been exposed to sarin or a sarin-like substance.

U.S. envoy to the United Nations Nikki Haley issued a written statement expressing confidence in the report, "which confirms what we already knew: chemical weapons were used against the Syrian people."

"Now that we know the undeniable truth, we look forward to an independent investigation to confirm exactly who was responsible for these brutal attacks so we can find justice for the victims," Haley added.

The Associated Press contributed to this report