Experts from the world's global chemical arms watchdog are continuing their mission to probe an alleged gas attack in Douma despite Western air strikes in Syria, the body said Saturday.
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.
May said the aim was to deter the Syrian authorities from further use of chemical weapons and to send a message to the wider world that it was unacceptable to use such weapons.
"Important infrastructure was destroyed which will result in a setback for the Syrian regime", Mattis said.
"We can not allow the use of chemical weapons to become normalised - within Syria, on the streets of the United Kingdom, or anywhere else in our world", she said.
Yesterday Prime Minister Theresa May insisted the military action was "legal" and defended the decision to go ahead without securing the backing of Parliament.
"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.
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"I have done so because I judge this action to be in Britain's national interest". Russian Federation and Syria claim the attack was fabricated.
Syrians flooded the streets to celebrate the triumph of the country's army against a US-UK-French coalition strike, teleSUR correspondent Hisham Wannous reported..
May held an emergency cabinet meeting to discuss possible action on Thursday and there had been calls for the British parliament to be consulted before any air strikes.
A YouGov poll for The Times newspaper this week indicated that only a fifth of voters believed that Britain should launch attacks on Syrian military targets and more than two-fifths opposed action.
Jeremy Corbyn, leader of the main opposition Labour Party, has questioned the legal basis for Britain's involvement.
Prime Minister Justin Trudeau said in a statement that Canada stands with its allies and that it supports the decision " to take action to degrade the Assad regime's ability to launch chemical weapons attacks against its own people".
Shortly after the military strikes were launched, Scotland's first minister Nicola Sturgeon said United Kingdom foreign policy should be set by Parliament and not Donald Trump after the U.S., United Kingdom and France bombed targets in Syria.