On Saturday, at least 40 people were killed in the attack on Syria's rebel-held town of Douma in Eastern Ghouta according to volunteer first responders and activists.
In the near term, the military strike appeared to ramp up a searing war of words between Washington and Moscow, which regarded the action as a attack on Russian Federation itself.
"The US - the possessor of the biggest arsenal of chemical weapons - has no moral right to blame other countries".
Maj. Gen. Igor Konashenkov, a Russian Defense Ministry spokesman, released statements by medics from Douma's hospital who said a group of people toting video cameras entered the hospital, shouting that its patients were struck with chemical weapons and causing panic. He added that the Russian military has proof of British involvement, but didn't immediately present it. He didn't elaborate or name the state.
As worldwide chemical weapons experts travelled to Syria to investigate a suspected gas attack by government forces which killed dozens of people, French President Emmanuel Macron spoke to Russia's Vladimir Putin and expressed concern about a worsening situation.
One senior official said Trump has asked his military to consider options that would include punishing Russian Federation and Iran, Syria's main foreign backers, in part over the president's growing level of exasperation with Putin.
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A fact-finding mission from the Organisation for the Prohibition of Chemical Weapons is expected to head to Douma.
Russian Foreign Minister Sergei Lavrov said earlier that Moscow was in contact with Washington to discuss an atmosphere he described as alarming.
During his meeting with Mr Trump and the chairman of the joint chiefs of staff Joe Dunford, Mr Mattis pushed for more evidence of the Assad regime's culpability for the attack to bolster the case for air strikes.
Moscow also said it was calling an emergency session of the United Nations Security Council, where it is a permanent member, to discuss the "aggressive actions" of the USA and its allies.
Speaking to reporters in Moscow, Lavrov reiterated a strong warning to the West against military action in Syria, saying any such "adventures" in Syria would increase flows of refugees into Europe.