A suspected chemical attack that killed at least 58 civilians in rebel-held northwestern Syria on Tuesday has prompted widespread outrage and calls for international action.
The Britain-based Syrian Observatory for Human Rights said an air strike in Idlib province which released "toxic gas" was likely carried out by government warplanes, a charge the regime denied.
UN Syria envoy Staffan de Mistura said the "horrific" attack was believed to be chemical and launched from the air, telling reporters in Brussels that there should be a "clear identification of responsibilities and accountability".
The Syrian opposition's chief negotiator at peace talks, Mohamad Sabra, said the attack cast new doubt on the UN-led peace process.
"If the United Nations cannot deter the regime from carrying out such crimes, how can it achieve a process that leads to political transition in Syria?" he said.
On Twitter, the head of the opposition High Negotiations Committee Riad Hijab said the "massacre is evidence that it is impossible to negotiate with a regime addicted to criminal behaviour".
A senior Syrian security source rejected allegations that Syria's government was behind the attack.
"This is a false accusation," the source said, adding that the opposition was attempting to "achieve in the media what they could not achieve on the ground".
Turkish President Recep Tayyip Erdogan told his Russian counterpart Vladimir Putin that "this kind of inhuman attack was unacceptable".
Presidential sources said Erdogan had told Putin by phone that the attack threatened peace talks in the Kazakh capital Astana, but did not assign blame for the attack.
German Foreign Minister Sigmar Gabriel said that if the Syrian regime had indeed carried out a chemical attack, "it would be an act of such cruelty as to be without equivalent", and "another reason we should not deal with the Assad regime in the fight against terrorism".
French President Francois Hollande accused the Syrian regime of a "massacre".
"Once again the Syrian regime will deny the evidence of its responsibility for this massacre," Hollande said in a statement.
"Those who support this regime can once again reflect on the enormity of their political, strategic and moral responsibility." France's Foreign Minister Jean-Marc Ayrault earlier called for an emergency United Nations Security Council meeting over the attack, which he described as "monstrous".
British Prime Minister Theresa May said she was "appalled" by reports of the attack, adding, "We condemn the use of chemical weapons in all circumstances." She added: "I'm very clear that there can be no future for Assad in a stable Syria ... and I call on all the third parties involved to ensure that we have a transition away from Assad.
"We cannot allow this suffering to continue." British Foreign Secretary Boris Johnson said in a statement that the incident bore the "hallmarks of an attack by the regime, which has repeatedly used chemical weapons".
European Union diplomatic chief Federica Mogherini said the Assad regime bore "primary responsibility" for the attack.
"Obviously there is a primary responsibility there of the regime because it has responsibility of protecting its people not attacking its people," she said.
Israeli Prime Minister Benjamin Netanyahu said the world must act to rid Syria of chemical weapons.
In a statement, he called on the international community "to fulfil its obligation from 2013 to fully and finally remove these horrible weapons from Syria".
Syria's government joined the Chemical Weapons Convention and agreed to give up its chemical arsenal in 2013 as part of a deal to avert US military action over previous alleged chemical attacks.
The United Nations' chemical arms watchdog said it was "seriously concerned" by the reports.
The Organisation for the Prohibition of Chemical Weapons (OPCW) said it was "gathering and analysing information from all available sources".
"The OPCW strongly condemns the use of chemical weapons by anyone, anywhere and under any circumstances," it added.