Experts from the U.N. and the chemical weapons watchdog are blaming the Syrian government for an attack in April using the nerve gas sarin that killed over 90 people.
Their report's key findings and conclusions, obtained by The Associated Press on Thursday, state that leaders of the expert body are "confident that the Syrian Arab Republic is responsible for the release of sarin at Khan Sheikhoun on April 4, 2017."
The report by the experts supports the initial findings by the United States, France and Britain that a Syrian military plane dropped a bomb with sarin on the town of Khan Sheikhoun.
Syria and Russia, its close ally, have denied any attack and strongly criticized the Joint Investigative Mechanism, known as the JIM, which was established by the U.N. and the Organization for the Prohibition of Chemical Weapons to determine responsibility for chemical weapons attacks in Syria.
The attack in Khan Sheikhoun sparked outrage around the world as photos and video of the aftermath, including quivering children dying on camera, were widely broadcast.
The United States blamed the Syrian military and launched a punitive strike days later on the Shayrat air base, where it said the attack was launched.
Responding to Thursday's report, U.S. Ambassador Nikki Haley said: "Today's report confirms what we have long known to be true. Time and again, we see independent confirmation of chemical weapons use by the Assad regime."
Clearly referring to Russia, she said, "In spite of these independent reports, we still see some countries trying to protect the regime. That must end now."
"The Security Council must send a clear message that the use of chemical weapons by anyone will not be tolerated, and must fully support the work of the impartial investigators," she said.
The experts also determined that the Islamic State extremist group was responsible for an attack in Um Hosh in Aleppo on Sept. 16, 2016 using mustard gas.
A fact-finding mission by the OPCW reported on June 30 that sarin was used in the Khan Sheikhoun attack and "sulfur mustard" in Um Hosh, but it was up to the JIM to determine responsibility.
The JIM report said its leadership panel "expresses its shock and dismay about the existence and use of these weapons in the Syrian Arab Republic, and its deep sympathy to those affected by them."
The report was issued two days after Russia vetoed a U.S.-sponsored resolution to extend the mandate of the JIM for another year.
Russia's U.N. Ambassador Vassily Nebenzia said Moscow wanted to wait for the JIM report.
Haley said countries that fail to support the JIM "are no better than the dictators or terrorists who use these terrible weapons."