The strike also raises thorny questions for Trump administration officials about why they are willing to intervene when Assad uses chemical weapons against civilians but won't act in instances where his forces are killing far more with conventional weapons.
It remains unclear whether the strike will prevent Assad's forces from turning to chemical weapons in the future as the leader seeks to extend his reach across the country while consolidating gains in the civil war. He said the fate of that region "lies in the hands of its own people".
"These attacks on multiple axes were able to overwhelm the Syrian air defence systems". She pointed out that the strikes took place before OPCW inspectors could carry out on-site inspections in Douma, tweeting: "Stop this madness!"
Theresa May declined to say whether Syrian President Bashar al-Assad should stay in power and said talks with allies would continue on finding a political solution to the civil war.
The Syrian foreign ministry said the US-led strikes would only "lead to inflaming tensions in the world" and threaten global security.
Damascus and Moscow deny blame for any such attack.
The Syrian government responded with defiance Saturday following President Trump's decision to take military action against chemical weapons production facilities in Syria, describing the attack as an "aggression" that would fail.
Mrs May said chemical weapons had "all too often" been used in recent times. But Friday night, the White House released what it said was "a significant body of information" that pointed to the use of chlorine gas - and possibly also the deadly nerve agent sarin.
Reading a statement on Syrian TV, Brig.
Trump has long criticized his predecessor Barack Obama for failing to enforce a "red line" in 2013 after earlier chemical attacks blamed on Assad's forces.
In Damascus, "hundreds of residents began gathering in the landmark Omayyad square of the Syrian capital".
"We are not looking for confrontation and refuse any logic of escalation, that is the reason why we, with our allies, ensured the Russians were warned beforehand", Parly said. He said the alleged attack had been "staged" by foreign intelligence services.
North Atlantic Treaty Organisation ally Turkey, which is waging its own military operation against Kurdish militias in northern Syria, also weighed in.
"I urge all member states to show restraint in these unsafe circumstances".
Nikki Haley, U.S. ambassador to the United Nations, said "the time for talk ended last night" with the allied attack on Syrian chemical weapons sites. US officials said they gave Russian Federation no specific warnings of the attacks or the targets, but used the usual hotline with Moscow's military to ensure the airspace was clear. "It is not about regime change", May said in statement made from her country residence at Chequers just minutes after U.S. President Donald Trump announced the strikes from the White House. Russian Federation has several military bases and thousands of troops in Syria, and the USA said Friday that it did not coordinate with or notify Russian Federation of the strikes.
Russia's ambassador to the U.S. responded by saying the attack on its ally "will not be left without consequences".
"All responsibility for them rests with Washington, London and Paris".
Surface-to-air missiles are seen over Syria's capital Damascus on April 14, 2018, as the Syrian air defenses were responding to USA attacks.