The Syrian government on Saturday dismissed a report by the international chemical weapons watchdog that said the banned nerve agent sarin was used in an April attack in northern Syria, saying it lacked “any credibility.”

Western governments including the United States have said the Syrian government carried out the attack in the town of Khan Sheikhoun, which killed dozens of people. The Syrian government has denied using chemical weapons.

The attack prompted a U.S. missile strike against a Syrian air base that Washington said was used to launch the strike. The report into the attack was circulated to members of the Organization for the Prohibition of Chemical Weapons (OPCW) in The Hague, but was not made public.

In a statement, the Syrian Foreign Ministry said the fact-finding team had based its report on “the testimonies offered by terrorists in Turkey.” Turkey is a major backer of the Syrian opposition to President Bashar al-Assad.

After interviewing witnesses and examining samples, the fact-finding mission of the OPCW concluded that “a large number of people, some of whom died, were exposed to sarin or a sarin-like substance.”

Russia, Assad’s most powerful ally, has described the report as biased.

The April 4 attack on in northern Idlib province was the most deadly in Syria’s civil war in more than three years. Western intelligence agencies had also blamed the Assad government. Syrian officials have repeatedly denied using banned toxins in the conflict.

Chlorine gas

A joint U.N. and OPCW investigation has found Syrian government forces were responsible for three chlorine gas attacks in 2014 and 2015 and that Islamic State militants used mustard gas.

Syria joined the chemical weapons convention in 2013 under a Russian-U.S. agreement, averting military intervention under then U.S. President Barack Obama.

The United States said Wednesday that the Syrian government appeared to have heeded a warning this week from Washington not to carry out a chemical weapons attack.

Russia warned it would respond proportionately if the United States took pre-emptive measures against Syrian forces after Washington said on Monday that it appeared the Syrian military was preparing to conduct a chemical weapons attack.
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