"We are still assessing the intelligence, ourselves and our allies, we are still working on this," Mattis told reporters when asked if he had seen enough evidence to blame President Bashar al-Assad's regime for the alleged chemical attack in Douma.

"We stand ready to provide military options, if they're appropriate, as the president determined."

President Donald Trump vowed on Twitter that missiles would be launched at Syria following Saturday's alleged chemical attack, all but guaranteeing a military strike against the regime of President Bashar al-Assad.

The Russian army has accused the White Helmets civil defense organization in Syria of staging a chemical weapons attack in Douma that has led to calls for further Western intervention.

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 the Organisation for the Prohibition of Chemical Weapons (OPCW) has been blamed for previous attacks.

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.