The international chemical weapons watchdog has confirmed sarin nerve gas was used in a deadly April 4 attack on a Syrian town, but a report stopped short of saying who was responsible.
The attack on Khan Sheikhoun in Idlib province killed more than 90 people, including women and children.
It sparked outrage around the world as photos and video of the aftermath, including quivering children dying on camera, were widely broadcast.
"I strongly condemn this atrocity, which wholly contradicts the norms enshrined in the Chemical Weapons Convention," said Ahmet Uzumcu, director-general of the Organisation for the Prohibition of Chemical Weapons.
"The perpetrators of this horrific attack must be held accountable for their crimes."
The US blamed the Syrian military for the attack and launched a punitive strike days later. Syrian President Bashar Assad has denied using chemical weapons.
The findings of the probe will be used by a joint United Nations-OPCW investigation team to assess who was responsible for the attack. The OPCW has scheduled a July 5 meeting of its executive council to discuss the matter.
The US State Department said: "The facts reflect a despicable and highly dangerous record of chemical weapons use by the Assad regime."
Assad's ally, Russian President Vladimir Putin, said earlier this month that he believed the attack was "a provocation" staged "by people who wanted to blame" the Syrian leader.
Russian foreign minister Sergey Lavrov said the report does not back claims by the US and its allies that the sarin was dropped from aircraft.
"They don't know how the sarin ended up there, yet tensions have been escalating for all these months," Mr Lavrov said in Moscow.
UK Foreign Secretary Boris Johnson said that while the report did not apportion blame, "the UK's own assessment is that the Assad regime almost certainly carried out this abominable attack".
The US and the OPCW defended the probe's methodology. Investigators did not visit the scene of the attack, deeming it too dangerous, but analysed samples from victims and survivors as well as interviewing witnesses.
The Syrian government joined the OPCW in 2013 after it was blamed for a deadly poison gas attack in a Damascus suburb.
As it joined, Assad's government declared 1,300 tons of chemical weapons and precursor chemicals that were subsequently destroyed in an unprecedented international operation.
However, the organisation still has unanswered questions about the completeness of Syria's initial declaration, meaning it has never conclusively been able to confirm that the country has no more chemical weapons.
The investigative team responsible for the report has previously concluded "with a high degree of confidence" that chlorine and sulphur mustard, commonly known as mustard gas, had been used as weapons in Syria.
