"It is about a limited and targeted strike that does not further escalate tensions in the region and that does everything possible to prevent civilian casualties", she said.
Britain's defence ministry said in a statement that four British Tornado jets had fired Storm Shadow missiles at the Syrian base 15 miles (24 kilometres) west of Homs at 0100 GMT.
Mattis noted that past year, US forces conducted a "unilateral strike on a single site", whereas he said these strikes "will result in a long term degradation to the Syrians' capability to research, develop, and employ biological and chemical weapons".
Britain's defense ministry said initial indications were that the precision weapons and meticulous target planning had "resulted in a successful attack". It is not about regime change.
Mr Johnson said action had to be taken against Syria for the Douma attack during an appearance on The Andrew Marr Show on Sunday.
The opposition leader questioned the prime minister's statement that the Syria attack is "right and legal".
"Given the context of the recent worldwide response to the use of a nerve agent in the United Kingdom, the clear targeted objective of the strikes, and the repeated blocking by Russian Federation of diplomatic solutions through the UN, we believe the Prime Minister was justified in standing with our American and French allies in this concerted action".
Earlier, the US, Great Britain and France claimed the strikes had been conducted in response to an alleged chemical weapons attack in Douma, a suburb of the Syrian capital.
Russia, which intervened in the war in 2015 to back Assad, has denied there was a chemical attack and has accused Britain of helping to stage the Douma incident to stoke anti-Russian hysteria.
British lawmakers voted down taking military action against Damascus in 2013, in what was widely viewed as an assertion of parliamentary sovereignty on the use of force.
Meanwhile, Britain's main opposition Labour leader Jeremy Corbyn described the US -led military strikes on Syria jointly staged by three leading Western countries as a "legally questionable action", saying that the British government should "not taking instructions from Washington".
"Bombing can not substitute for diplomacy", he said.
He reiterated that Canada condemns the use of chemical weapons in Ghouta.
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