Paris - 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 the 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.
"What we have understood, it was a chemical attack and it came from the air," de Mistura told reporters in Brussels, adding 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".
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.
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," he added.
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 Foreign Secretary Boris Johnson condemned the attack and said it bore the "hallmarks" of a regime action.
"Horrific reports of chemical weapons attack in Idlib, Syria. Incident must be investigated and perpetrators held to account," he wrote on Twitter.
In a separate statement, he added: "This bears all 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.