But Trump reportedly also followed up the warning with communications to Russia to help avoid US missiles hitting Russian targets in Syria, despite the White House's statement that the US wasn't afraid to hit them.
It may be impossible to prevent Assad from resorting again to the barbarism that helped him and his father retain power in Syria.
In London, British Prime Minister Theresa May said all indications pointed to Syrian government responsibility for the Douma attack and such "a shocking and barbaric act" could not go unchallenged. Outside meddling in the Middle East's civil wars has never been productive, except of death and destruction.
Russian military said it had observed movements of US Navy in the Gulf. "The president holds Syria and Russia responsible for this chemical weapons attack".
The Syrian government has repeatedly refuted allegations of any involvement in alleged chemical attacks, referring to the fact that it destroyed all its chemical weapons stockpiles several years ago under the supervision of the Organization for the Prohibition of Chemical Weapons (OPCW).
The US ambassador to the UN, Nikki Haley, quickly labeled Assad a "monster" following reports of the alleged attack in Douma.
Trump said the U.S. will respond to the Saturday attack within 48 hours at most, but a possible military move has been slowed down as the White House seeks to coordinate it with its allies, including France and the U.K.
The World Health Organisation also demanded "immediate unhindered access" to Douma, saying an estimated 500 patients went to health facilities with "signs and symptoms consistent with exposure to toxic chemicals" after the suspected attack. Regimes fighting for their existence do not care about condemnation or the niceties of international treaties.
At another point, Trump blamed the bad relationship on Special Counsel Robert Mueller and his investigation into links between the Trump campaign and Russians who sought to influence the 2016 presidential election through hacks and fake news. Officials said Thursday the trio has yet to reach firm agreement on the scale or timing of a response.
Targeted economic sanctions against Syria and its allies, Iran and Russia could still be used. The US bombs would be "nice and new and 'smart'", he added.
Syrian government forces have regained full control of the city as the international community considers its response. "This posturing may trigger a dangerous process", Pushkov said on Thursday at a meeting with representatives for the Association of Foreign Correspondents at the Federation Council.