Assad, who denies using chemical weapons, said the West had waged a campaign of "lies and misinformation", when he spoke to visiting Russian politicians on Sunday.
And in a phone call with Iran's leader Hassan Rouhani, Vladimir Putin warned any further US-led strikes could provoke "chaos in international relations", while the leaders of both countries called it an illegal "act of aggression".
The US will on Monday announce further economic sanctions on Russia for its support of the Syrian regime, with the sanctions affecting any country "dealing with equipment related to Assad and any chemical weapons use," according to US ambassador to the UN Nikki Haley.
Haley told Fox News Sunday: "Mission accomplished is a military term, and as a military spouse I know that mission accomplished means you have one task currently in front of you and when it's completed, it is mission accomplished.
"Politically, mission accomplished means something broader. And I think that the president was referring in military terms. We of course know that our work in Syria is not done. We know that it is now up to Bashar al-Assad on whether he's going to use chemical weapons again and should he use it again, the President has made it very clear that the United States is locked and loaded and ready to go."
Pentagon spokeswoman Dana White insisted: "We met our objectives. We hit the sites, the heart of the chemical weapons programme. So it was mission accomplished.
"What happens next depends on what the Assad regime decides to do."
'PARTS, BUT NOT THE HEART'
Senior Pentagon official Lieutenant General Kenneth McKenzie said the targets were "fundamental components of the regime's chemical weapons warfare infrastructure."
But the New York Times said that while they may have been in the past, it was unclear whether they were still in active use when they were bombed by the West, since there were no casualties and no reports of chemical leakages at the sites.
Lt Gen McKenzie admitted that while he believed "we took the heart of it out", there was "still a residual element" of the program left. "I'm not going to say that they are going to be unable to continue to conduct a chemical attack in the future," he said, but claimed they would "think long and hard about it."
A former officer of the program, however, said Saturday's co-ordinated attack had probably hit "parts of, but not the heart" of Mr Assad's chemical facilities.
Captain Adulsalam Abdulrazek, who defected in 2013 when there were an estimated 50 warehouses storing chemical weapons, told the Associated Press that many of these were never dismantled, despite promises by Syria to the US. Even US officials confessed it was "highly likely" Assad still had a stockpile and the means to manufacture the chemicals.
The Syrian leader does not even need sophisticated production facilities — experts believe the regime may have used chlorine to gas civilians, in a sickening attack that killed at least 34 people. Yellow canisters were photographed at the scene of the devastation, almost hidden on street strewn with bodies.
Chlorine is widely available in most countries because of its importance for water purification.
Syria has deep knowledge of chemical agent production, so even if Mr Assad has lost some key production sites, he could easily rebuild them elsewhere.
A French intelligence report released on Saturday noted that the nerve agent sarin could be produced anywhere, adding, "the Syrian military retains expertise from its traditional chemical weapons agent program to both use sarin and produce and deploy chlorine munitions."
And America appears to be aware of this too. The French report said the US "also assesses the regime still has chemicals — specifically sarin and chlorine — that it can use in future attacks."
'CATASTROPHIC SUCCESS'
This is the not the first time the US has attacked Syria following a chemical strike on its people. Trump did the same thing a year ago, although this bombing was twice the size of the last. There is no reason to believe this will have any more effect, despite the rhetoric from the US government.
Assad has made a show of being utterly unperturbed by the reported 100 missiles.
Russia and Syria claimed they had shot down dozens of the missiles, although Lt Gen McKenzie said Putin's troops did not fire, and Syria's attempts were completely unsuccessful.
Now, experts fear what the next attack could have in store. The Syrian government could destroy an entire community in its efforts to spread fear and remove the rebel element that has driven a bloody seven-year civil war.
Kings College research associate Eliot Higgins noted that the high number of deaths most likely wasn't planned. He said the canister appeared to have been dropped at random, and had just happened to land on a densely packed building. Had it fallen elsewhere, "we'd see a fraction of deaths, and you probably wouldn't have even heard about it happening," he said. It was only because of the attack's "catastrophic success" that it had garnered so much attention.
He noted that the chemical attack on Khan Sheikhoun last year was preceded by another sarin attack just days earlier — but no one heard about it since there weren't many casualties.
There is no reason to believe the Western strike will stop Assad, and no reason to believe that anything has been achieved other than stoking global tensions.
