Condemnation mounted over a chemical attack in Syria that killed at least 70 people on Tuesday and activists and various nations rejected Russian claims that a Syrian airstrike on a "terrorist" ammunition depot caused the deaths.

The United Nations Security Council is due to hold emergency talks later following the release of gas in Khan Sheikhoun in Idlib province which - according to UK-based monitoring group the Syrian Observatory for Human Rights - killed 72 people, including 20 children.

Dozens of people were treated for symptoms of chemical poisoning, including foaming at the mouth and suffocation, said a CNN report.

Activists said the Syrian regime dropped a chemical bomb and was responsible for the killings.

Britain's foreign secretary, a rebel commander and a weapons expert all said the evidence pointed to an attack by Syrian government forces, said a BBC report.

British Foreign Secretary Boris Johnson said all evidence suggested that the Assad regime was behind a "barbaric act."

If the incident is confirmed to be a chemical weapons attack, it would be one of the deadliest of its kind since the Syrian war began six years ago.

Syria denies the accusations and Russia says it had no warplanes in the vicinity.

The attack coincided with a two-day meeting in Brussels on Syria's future. Seventy donor nations are discussing aid efforts in the war-ravaged country.

The Russian defence ministry claimed on its Facebook page that a Syrian airstrike hit "workshops, which produced chemical warfare munitions" in the eastern outskirts of Khan Sheikhoun.

It said that "terrorists" had been transporting the chemical munitions from its largest arsenal to Iraq.

But a chemical weapons expert, Col Hamish de Bretton-Gordon, told the BBC's Radio 4 that all signs showed the chemical used was sarin gas and that Russia's versions of events was "completely unsustainable."

"I think this is pretty fanciful and no doubt the Russians trying to protect their allies. Axiomatically, if you blow up sarin you destroy it," he said.

In a statement on Tuesday, US President Donald Trump condemned what he called "these heinous actions" by the government of President Assad.