Syria has denied using any chemical agents, and Russia says the US has failed to provide evidence Syria has chemical weapons.
Russia and Iran, President Assad’s key military backers, are also threatening retaliation if there are any further American air strikes, saying the US attack had crossed “red lines”.
“From now on we will respond with force to any aggressor or any breach of red lines from whoever it is and America knows our ability to respond well,” said a statement on Sunday from a joint command centre comprising the forces of President Assad’s allies.
G7 nations are meeting in Italy to try to hammer out a unified approach to the Syria conflict, in the wake of last week’s suspected chemical attack.
Foreign ministers will seek to pressure Russia to distance itself from Syrian President Bashar al-Assad.
UK Foreign Secretary Boris Johnson said possible sanctions against Russian military officials would be discussed.
Allies will also be seeking clarity from the US on its Syria policy, after some apparently mixed messages.
US Secretary of State Rex Tillerson has strongly criticised Russia for failing to prevent Syria from carrying out the chemical attack on the rebel-held town of Khan Sheikhoun last Wednesday that left 89 people dead.
But he also said there had been “no change to our military posture” in Syria following a retaliatory US strike against a Syrian airbase, and that Washington’s “first priority” in Syria was to defeat so-called Islamic State (IS).
What is US policy on Syria?
That is what the other G7 ministers will be seeking to clarify.
In addition to Mr Tillerson’s comments, the US ambassador to the United Nations, Nikki Haley, appears to have given mixed signals.
“In no way do we see peace in that area with Assad as the head of the Syrian government,” she told NBC’s Meet the Press, a week after she had said that Mr Assad’s removal was no longer a US priority.
White House sources said any difference in nuance between her comments and administration policy was “inadvertent and unintentional”.
Firing 59 cruise missiles at Syria’s Shayrat airbase following the suspected chemical attack was the kind of direct intervention in Syria’s six-year-old civil war that Donald Trump’s predecessor, Barack Obama, had avoided.
Map showing location of Shayrat air base
Until Friday’s strike, so did Mr Trump. He opposed military action in 2013 and campaigned on a platform of fixing problems in the US, not Syria.
During the election campaign he expressed wariness that US military action in Syria might lead to conflict with Russia.
And while viewing the airbase strike as decisive, most commentators are not seeing a radical change in the US approach to Syria.