The chemical used in an attack that killed scores of Syrian civilians was most likely the deadly nerve agent sarin, the Turkish Health Ministry said Thursday.
Autopsies conducted on three victims by Turkish doctors confirmed that chemical weapons were used in a daybreak strike on Tuesday widely attributed to the Syrian government, providing the most concrete evidence to date for why so many people died.
"According to the preliminary results, the findings suggest that the patients were exposed to a chemical substance (Sarin)," the statement said.
Dozens of victims of the attack on the northwestern town of Khan Sheikhoun have been evacuated to Turkey for medical treatment. Turkish Justice Minister Bekir Bozdag said that the World Health Organization supervised the autopsies and that the results were sent to The Hague for further analysis.
In August 2013, the Syrian government's use of sarin in an attack on a densely populated Damascus suburb killed hundreds of civilians, many as they slept.
At least 70 people were killed in Tuesday's attack, which witnesses described as a fog of chemicals that enveloped men, women and children, leaving many to suffocate, choke or foam at the mouth. In Khan Sheikhoun, a list of names compiled by residents put the death toll at 83.
Syrian Foreign Minister Walid al-Moualem denied Thursday that the government had used chemical weapons in the past and maintained that it never would. Speaking in Damascus, the capital, he rejected the findings of the Turkish investigation, saying that Syria's experience with international inquiries has not been encouraging and insisting that a credible investigation into the attack must begin from Damascus, not Turkey.
Under the conditions of a 2013 deal brokered by Russia - one of President Bashar al-Assad's main backers - Syria was required to declare the chemical weapons in its possession and hand its stockpiles over for destruction.
Chemical weapons inspectors have visited Syria on a number of occasions since then. Although the delegation's movements have been heavily constrained by the Syrian government, experts from the Organization for the Prohibition of Chemical Weapons said in May 2015 that they had found traces of chemicals used to make sarin and the nerve agent VX at a military research site.
The global chemical weapons watchdog said Thursday that it has "initiated contact" with Syrian authorities as it investigates the attack. In a statement reported by the Associated Press, the organization said it has been "collecting and analyzing information" as part of its ongoing fact-finding mission in Syria.
On Wednesday, President Donald Trump accused Assad's government of going "beyond a red line" with the attack on civilians, and he suggested that his anti-interventionist stance toward the conflict may be changing.
But there were few indications of what that might mean in practice. Syria's complex conflict has paralyzed a divided U.N. Security Council and left Western leaders reluctant to face the possible consequences of military intervention against Assad.
President Vladimir Putin's spokesman said Thursday that Russia's support for Assad is not "unconditional" but called for a full investigation of Tuesday's attack before the United Nations takes any action.
The incident puts Moscow in a tough spot: Condemning Assad could deal the final blow to Putin's carefully assembled peace process, which brings together Turkey and Iran - regional powers that have backed opposing sides in the civil war. At the same time, the attack could indicate that Assad and his Iranian allies have no intention of being party to a power-sharing agreement, suggesting that Putin's deal is all but dead.
The top U.N. humanitarian envoy for Syria, Jan Egeland, called the attack a possible "watershed moment" that could force world leaders to take stronger action to ease the "suffering of the civilians that we see every day."
In an interview published Thursday, however, Assad insisted that a military victory was the government's only option. "We have no choice in facing this war, and that's why we are confident, we are persistent, and we are determined," he told Vecernji List, a Croatian newspaper. His comments appeared to have come before Tuesday's attack.
The Washington Post's Zakaria Zakaria in Antakya and Brian Murphy in Washington contributed to this report.