"This action risks not just further escalating the civil war in Syria but also a risky escalation of global tensions", said the leader of the left-wing Scottish National Party, the third-biggest force in the British parliament.
The ministers said there was a need to "deter the further use of chemical weapons by the Assad regime", and alleviate the humanitarian situation, a Downing Street statement said. The mission had been a success, she said.
Mattis had reportedly been arguing for a cautious response that would minimise the risk of the USA being dragged deeper into Syria´s civil war.
The next few days will show whether Britain is being dragged into a far deeper and deadly conflict, a British academic said Saturday in response to the US -led strikes on Syria.
Mattis tells reporters he is certain Assad conducted a chemical attack on innocent people.
"Air strikes by U.S. and United Kingdom forces have not resolved the situation in Syria in the past and I am not persuaded they will do so now".
Opposition Labour leader Jeremy Corbyn said it is "vital that parliament has the chance to debate and decide in advance" of any military action, which he warned "risks a unsafe escalation of the conflict".
If so, last night's action will be seen as one of the more successful military interventions of recent times; if not, expect further strikes.
The General command stressed its determination in defending Syria and protecting its citizens, reiterating that such aggression will not affect the Syrian armed forces to continue hunting the remnant of armed terrorist gatherings across Syria.
Middle Eastern affairs expert and KUSI contributor Tony Perry gives his thoughts on the strikes on Syria, if the strikes could take Syrian President Bashar al-Assad out of the chemical weapon business, the United States' objective in Syria, and Russia's response to the missile strikes.
The Cabinet has been meeting in Downing Street to discuss Britain's response to a suspected chemical attack.
Friction between Britain and Russia has escalated following the poisoning of Russian ex-spy Sergei Skripal, 66, and his daughter Yulia Skripal, 33.
She said nearly a century of global acceptance about not using chemical weapons had been eroded in Douma and Salisbury.
"This collective action sends a clear message that the worldwide community will not stand by and tolerate the use of chemical weapons", May told a press conference.
Sky News reported that May was expected to ask her ministers on Thursday to approve Britain's involvement in military action against Syria's chemical weapons infrastructure. May has a slim majority thanks to a deal with a small Northern Irish party to support her Conservatives.
"Bombing can not substitute for diplomacy, military reactions a substitute for a peace plan". But 46% still believed she was better than Jeremy Corbyn on dealing with an worldwide crisis, with just 26% backing the Labour leader.
Former Prime Minister David Cameron lost a parliamentary vote on air strikes against Assad's forces in 2013 when 30 Conservative lawmakers voted against action, with many Britons wary of entering another conflict after intervention in Iraq, Afghanistan and Libya failed to bring stability to the region.
And reliable intelligence indicates that Syrian military officials co-ordinated what appears to be the use of chlorine in Douma on 7 April.
Formally, the prime minister has the right to go to war without approval from parliament, but a convention has been established in previous conflicts where MPs have a vote either before or shortly after military action begins.
North Atlantic Treaty Organisation allies have given the action their full support, Secretary General Jens Stoltenberg said.