British and French leaders have spoken in unity with the US president, following the announcement the two countries would join the US in military strikes on Syria.
British Prime Minister Theresa May said Saturday there was "no practicable alternative" to the use of force in Syria as she announced Britain had joined France and the United States in launching strikes against Syria.
"This evening I have authorised British armed forces to conduct co-ordinated and targeted strikes to degrade the Syrian regime's chemical weapons capability and deter their use," she said in a statement.
Mrs May said "a significant body of information including intelligence" pointed to Syrian government responsibility for a suspected chemical attack last Saturday.
"There is no practicable alternative to the use of force to degrade and deter the use of chemical weapons by the Syrian regime," she said.
"This is not about intervening in a civil war. It is not about regime change.
"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.
In a second press conference later on Saturday, AEST, Mrs May said the strikes had sent a "clear message".
This collective action sends a clear message that the international community will not stand by and tolerate the use of chemical weapons," Mrs May told a press conference.
She said that at an emergency cabinet meeting in London on Thursday "we agreed that it was both right and legal to take military action" after hearing legal advice.
"I believe that the action taken will have significantly degraded the Syrian regime's ability to use chemical weapons," she said.
"While the full assessment of the strike is ongoing, we are confident of its success," she added.
Macron: We cannot tolerate the normalisation of the use of chemical weapons
French President Emmanuel Macron said that France had joined the US and Britain in an ongoing operation of strikes to target "the capacities of the Syrian regime to produce and use chemical weapons".
"We cannot tolerate the normalisation of the use of chemical weapons," he said in a statement issued shortly after huge explosions were heard in Syria's capital early Saturday followed by the sound of airplanes overhead.
For Mr Macron, "the facts and the responsibility of the Syrian regime are not in doubt," concerning the "deaths of dozens of men, women and children" in what he said was a chemical weapons attack on April 7 in Douma.
"The red line set by France in May 2017 has been crossed," he said.
"I have ordered the French army to intervene tonight as part of the international operation in coalition with the US and Britain directed against the secret chemical arsenal of the Syrian regime," he said.
Additional reporting SBS News