In photos: Syria chemical weapons attack death toll rises to 72

Many witnesses claim that the toxic substance spread after warplanes dropped bombs in the early morning hours.

The US government believes the chemical agent sarin was used in the attack, a US government source said, adding that it was "almost certainly" carried out by forces loyal to Syrian President Bashar al-Assad.

US President Donald Trump denounced the carnage as a “heinous” act that “cannot be ignored by the civilised world”. But he also laid some of the responsibility on Barack Obama, saying in a statement that the attack was “a consequence of the past administration’s weakness and irresolution”.

UK PM Theresa May said she was appalled by reports of the attack and called for an investigation by the Organisation for the Prohibition of Chemical Weapons. “I’m very clear that there can be no future for Assad in a stable Syria which is representative of all the Syrian people 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,” she said.

The United States, Britain and France on Tuesday proposed a United Nations Security Council resolution condemning the attack, which they have blamed on Assad's forces. Diplomats said the resolution would likely be put to a vote on Wednesday.

The Syrian military, for its part, denied responsibility and said it would never use chemical weapons, echoing denials it has made over the course of the six-year-long Syrian civil war that has killed hundreds of thousands, created the world's worst refugee crisis, and drawn in nations such as Russia, Iran and the United States.

Russia’s defense ministry said Wednesday that a Syrian airstrike targeting militant workshops producing chemical weapons was responsible for the deaths.

According to a BBC report, the Russian defence ministry acknowledged that Syrian planes had attacked the town of Khan Sheikhoun in Idlib province. But it said the aircraft had struck a depot producing mines filled with a poisonous substance being made for use in Iraq.

If confirmed, the incident in Idlib would be the deadliest chemical attack in Syria since a sarin gas attack that killed hundreds of civilians in Ghouta near Damascus in August 2013.