The suspected chemical attack occurred on April 7 in the Damascus suburb of Douma, which had been held by rebel forces for years in Syria's continuing civil war.
Independent inspectors are attempting to assess the attack this weekend. It has been used repeatedly before by the regime but does not usually cause heavy casualties. It was not known if U.S pilots were flying over Syrian airspace, however, Mattis did say "manned aircraft" were involved in the airstrikes.
A USA -led coalition has been conducting airstrikes in Syria since September 2014 as part of a largely successful effort to break the IS grip on both Syria and Iraq.
A Syrian military statement says the U.S., Britain and France fired 110 missiles during a joint attack on targets in Damascus and outside.
Britain's defence ministry said that four Tornado jets fired Storm Shadow missiles, while the French defence ministry released video footage of Rafale warplanes taking off to carry out the strikes. She said of the alleged chemical attack that "Russia can complain all it wants about fake news, but no one is buying its lies and its cover-ups". A USA military response was widely expected to include missile strikes.
Russian officials quickly weighed in on the strike.
She blamed "all those who claim moral leadership in this world, who declare their exceptionalism" for the attack, noting that "you should be really exceptional" to attack Syria at a time "when it was given a chance for a peaceful future". The combined forces are prepared to respond until Syria no longer use chemical weapons on its citizens.
Allegations of Assad's chlorine use are frequent in Syria's conflict, raising questions about whether Washington had lowered the threshold for military action in Syria by now deciding to strike after a chlorine gas attack.
"We can not tolerate the recurring use of chemical weapons, which is an immediate danger for the Syrian people and our collective security", a statement from the Elysee presidential office said. He said it would be "a red line for us".
More than 40 people were believed killed.
Obama gave a speech saying the US should act against Syria. With the extremist group largely defeated, the president says he supports the withdrawal of US forces. They also raised the possibility of confrontation with Russian Federation and Iran, both of which have military forces in Syria to support Assad.
But the OPCW is not mandated to ascribe blame to any party, simply to establish facts like the kind of agent that was used, how it was delivered and how many people it killed or wounded.
On Friday, the Russian Defense Ministry stated that it had proof that the "chemical attack" in Douma had been a provocation and had been staged by Western-backed NGOs, including the White Helmets.
"It's.highly likely an attempt to create complications for the Organisation for the Prohibition of Chemical Weapons (OPCW) mission which was just starting its work in Syria's Douma, or an attempt to completely derail it", Interfax quoted Kosachev as saying.