Moscow on Thursday called on the West to "seriously consider" the consequences of threats against Syria after the USA and France said they would respond to an alleged chemical attack.
President Trump says he'll make a decision "fairly soon" on whether to launch a missile attack on Syria.
According to media reports, a Russian official said that Russia is ready to shoot down any and all missiles fired at Syria by the United States, while the USA side threatened military options at any time.
At stake in Syria is the potential for confrontation, if not outright conflict, between the US and Russian Federation, former Cold War foes whose relations have deteriorated in recent years over Moscow's intervention in Ukraine, its interference in the 2016 USA presidential election and its support for Syrian President Bashar Assad.
President Donald Trump on Thursday softened his rhetoric about potential airstrikes on Syria, a day after warning Russian Federation that missiles "will be coming, nice and new and 'smart'".
The U.S. officials told the American news outlet that the Assad regime is known to have used a mixture of chlorine and the nerve agent sarin in previous attacks. "Following a discussion in which every member present made a contribution, cabinet agreed it was vital that the use of chemical weapons did not go unchallenged", it read.
Asked whether the USA military was ready to conduct an attack in Syria if ordered, Mattis replied, "We stand ready to provide military options if they're appropriate, as the president determined".
The Syrian government denies responsibility.
"The chemical weapons attack that took place on Saturday in Douma in Syria was a shocking and barbaric act", May told reporters on Wednesday.
But across Western capitals opposition to military action also grew.
"You are a minority Government, and you need to seek the consent of Parliament before you commit the United Kingdom to any action".
Asked about the risks of US military retaliation, Mattis cited two concerns, starting with avoiding civilian casualties. "We need to know where that's going, what the objective of it is before we take that act".
Echoing the USA stance, France said Assad's government had reached a "point of no return" with repeated use of chemical weapons.
The United States, Britain and France made the case at the United Nations on Friday for military action against Syria, saying President Bashar al-Assad's repeated use of chemical weapons posed a threat to the world.
A separate YouGov survey on Thursday found 61 percent of people think it would be necessary for parliament to vote on military action against Syria, with just 18 percent saying it was not necessary and 21 percent undecided.
Post-World War II Germany typically has been reluctant to engage in military action, and parliamentary approval is required for any military missions overseas. May isn't legally required to do that, though it has become conventional since the 2003 invasion of Iraq for lawmakers to be given the chance to vote before British forces are deployed.
France already has some 1,100 troops involved in its Operation Chammal, created in 2014 to fight Islamic State extremists in Iraq and extended in 2015 to Syria, as part of the USA -led coalition. Russian Federation has blamed Syrian opposition forces for the attack.
Britain has launched air strikes against Islamic State militants in Syria, but not against the country's government.