Turkey and the World Health Organisation say victims from a suspected toxic gas attack died of chemical poisoning and were exposed to substances found in chemical weapons.
Postmortem results have revealed that chemical weapons were used in an attack in Syria's Khan Shaykhun, Turkish Justice Minister Bekir Bozdag said on Thursday.
Doctors from the World Health Organisation (WHO) who carried out the postmortems said victims showed symptoms consistent with reactions to a nerve agent.
"Some cases appear to show additional signs consistent with exposure to organophosphorus chemicals, a category of chemicals that includes nerve agents," WHO said in a statement.
WHO added victims had no apparent external injuries and died from a rapid onset of similar symptoms, including acute respiratory distress.
It said its experts in Turkey were giving guidance to overwhelmed health workers in Idlib on the diagnosis and treatment of patients, and medicines.
Representatives of the United Nations Organisation for the Prohibition of Chemical Weapons also participated in the postmortems.
Turkey's health minister said the postmortem results will be examined in The Hague after Turkey.
The suspected chemical attack killed more than 100 people, Turkish President Tayyip Erdogan said.
Thirty-two victims of Tuesday's attack were brought to Turkey, but three have subsequently died.
The US says the deaths were most likely caused by sarin nerve gas dropped by Syrian regime aircraft.
TRT World's Ediz Tiyansan reports from Hatay on Turkey's border with Syria.
Russia & regime deny Syrian regime is responsible for chemicals
Regime backer Russia said it believes poison gas had leaked from an opposition chemical weapons depot struck by Syrian bombs, adding the rebels had used the same chemical weapons in Aleppo last year.
The regime denied responsibility for the attack and says it does not use chemical weapons. The regime's foreign minister on Wednesday said ex-Al Qaeda affiliate Al Nusra Front and Daesh have been storing chemical weapons brought from other countries in urban and residential areas in Syria.
International concern at latest developments
US President Donald Trump accused the Syrian regime of going "beyond a red line" with the poison gas attack on civilians.
He said his attitude toward Syria and Assad had changed.
Also, US Vice President Mike Pence said on Wednesday that "all options are on the table" for a US response to the attack.
"We think that it is time that the Russians really need to think carefully about their continued support of the Assad regime," said US Secretary of State Rex Tillerson on Wednesday.
The US warned that when the UN fails to act collectively then states "are compelled to take our own action."
"When the United Nations consistently fails in its duty to act collectively, there are times in the life of states that we are compelled to take our own action," US UN Ambassador Nikki Haley told a Security Council meeting on Syria.
British Foreign Secretary Boris Johnson said on Thursday a UN resolution should be passed before any unilateral action is taken in Syria.
"I cannot understand how anybody on the UN Security Council could fail to sign up to a motion condemning the actions of the regime that is almost certainly responsible for that crime," Johnson said.
France is still seeking the UN Security Council resolution on Syria, Foreign Minister Jean-Marc Ayrault said on Thursday, adding that diplomatic negotiations were a priority over possible military action.
"France is still seeking to talk with its partners on the Security Council, especially the permanent members, and Russia in particular," Ayrault said.
China's permanent representative to the UN, Liu Jieyi, on Wednesday called for a comprehensive, objective and fair investigation into uses of chemicals as weapons in Syria.
The UN Commission of Inquiry into Human Rights in Syria has previously said the regime has used lethal chlorine gas on multiple occasions.
Hundreds of civilians died in a sarin gas attack in Ghouta on the outskirts of Damascus in August 2013. But the regime has always denied responsibility for that attack.
Syrian regime says no "option except victory"
The regime leader Bashar al Assad said there is no "option except victory" in the country's civil war in an interview published on Thursday.
He said the regime could not reach "results" with opposition groups that attended recent peace talks.
"If we do not win this war, it means that Syria will be deleted from the map. We have no choice in facing this war, and that's why we are confident, we are persistent and we are determined," he said.