"We are not surprised by such a reckless escalation from a regime like the United States which has fostered and continues to foster terrorism in Syria," SANA reported an official source at the foreign ministry as saying.

Damascus, which refers to all rebels as "terrorists", has almost entirely expelled opposition fighters from their former bastion of Eastern Ghouta outside Damascus since mid-February.

Trump said Wednesday that "missiles will be coming" in response to an alleged chemical attack in Ghouta's last rebel holdout of Douma at the weekend.

"It's not at all strange that it (the United States) supports the terrorists in Ghouta and condones their fabrications and lies to use them as a pretext to target Syria," the official source said.

Trump and other Western leaders have vowed a quick and forceful response to Saturday's alleged gas attack, which rescue workers say killed more than 40 people.

The United States, Britain and France have argued the attack bears all the hallmarks of a strike ordered by the regime, which has been blamed for previous attacks by the Organisation for the Prohibition of Chemical Weapons (OPCW).

Damascus and its ally Moscow have rejected the accusations as "fabrications".

Last year, Trump launched a cruise missile strike against a Syrian air base in retaliation for a sarin attack the United Nations later pinned on Assad.

More than 350,000 people have been killed in Syria's conflict since it started in 2011 with the brutal repression of anti-government protests.

It has since spiralled into a complex war involving world powers.