US President Donald Trump on Sunday said there will be a “big price to pay” after what he called a “mindless CHEMICAL attack” in Syria, allegedly involving chlorine gas.

Trump also called Syrian President Bashar al-Assad an “animal.” “President Putin, Russia and Iran are responsible for backing Animal Assad.

"Big price to pay,” Trump said in a pair of tweets which began with a discussion of the attack in Syria's Eastern Ghouta, where rescue workers alleged that regime loyalists had used chlorine gas.

“Many dead, including women and children, in mindless CHEMICAL attack in Syria. Area of atrocity is in lockdown and encircled by Syrian Army, making it completely inaccessible to outside world,” the president said.

At least 80 civilians have been killed since Friday after the regime launched fresh air raids on rebel-held areas of Eastern Ghouta, according to the Syrian Observatory for Human Rights, a Britain-based monitor.

Syrian state media and the regime's ally Russia denounced claims of chemical use as “fabrications.” “Open area immediately for medical help and verification,” Trump said.

“Another humanitarian disaster for no reason whatsoever. SICK!”

The latest alleged attack came a year after the rebel-held town of Khan Sheikun in northwestern Syria was hit by an air strike. A UN-commissioned report said many residents of the town suffered the symptoms of an attack from an illegal nerve agent and more than 80 or them died, convulsed in agony.

Trump responded to that attack three days later, when US warships in the Mediterranean fired 59 cruise missiles at a Syrian airbase.

Assad denied ordering that attack and Russia has continued to give him diplomatic cover at the United Nations.

Trump on Sunday criticised his predecessor Barack Obama for not striking after warning that the use of chemical weapons in Syria was a “red line.”

“If President Obama had crossed his stated Red Line in The Sand, the Syrian disaster would have ended long ago! Animal Assad would have been history!” Trump said.