Several huge explosions were heard in Syria's capital Damascus as US President Donald Trump announced strikes on the country.
Mr Trump said he ordered precision strikes on targets of chemical weapons capability in Syria. Western strikes have hit Syrian military bases and chemical research centres in and around the capital, a monitor said.
"The Western coalition strikes targeted scientific research centres, several military bases, and the bases of the Republican Guard and Fourth Division in the capital Damascus and around it," said the Britain-based Syrian Observatory for Human Rights.
Speaking on Saturday AEST, Mr Trump said a combined operation led by US with France and UK against the Assad regime is "now underway".
Mr Trump announced that a joint US-British-French operation had been launched on Syria, targeting the "criminal" regime of Bashar al-Assad and saying an alleged chemical attack had marked a "significant escalation."
"A short time ago, I ordered the United States armed forces to launch precision strikes on targets associated with the chemical weapons capabilities of Syrian dictator Bashar al-Assad," Trump said in a primetime televised address to the nation.
"A combined operation with the armed forces of France and the United Kingdom is now underway. We thank them both. This massacre was a significant escalation in a pattern of chemical weapons use by that very terrible regime."
Syrian state television also reported a US attack on Syria, in coordination with France and Britain.
Mr Trump had warned on Wednesday that missiles "will be coming" in response to the Douma incident.
Russia told the United States and its allies not to carry out any military strike.
Mr Trump also warned Russia and Iran not to stand by their ally in Damascus.
"Russia must decide if it will continue down this dark path or if it will join with civilized nations as a force for stability and peace," he said.
The decision signals a new chapter in a brutal seven-year-old civil war.
President Donald Trump speaks about the United States' military response to Syria's chemical weapon attack on April 7.
AAP
Britain's Prime Minister Theresa May was more cautious, but by the time the first precision cruise missile was launched, Mr Trump had a mini-coalition.
Mrs May said there was "no practicable alternative" to the use of force against Mr Assad.
Mr Trump's language was more brutal: "They are crimes of a monster."
Pentagon chief says strikes 'carefully calibrated'
A quick series of US, French and British airstrikes in Syria were designed to send a "clear message" to President Bashar al-Assad and stop his chemical weapons program, US Defense Secretary Jim Mattis said Friday.
The Pentagon chief stressed the strikes were carefully calibrated to provide a strong response to a suspected chemical attack, but avoid pulling the West into Syria's civil war.
"The targets tonight again were specifically designed to degrade the Syrian war machine's ability to create chemical weapons and to set that back," Mr Mattis said.
"There were no attempts to broaden or expand that target set."
The air strikes at multiple targets began around 9:00 pm Eastern Time (0100 GMT) and saw US, French and British assets firing missiles at the Assad regime's chemical production facilities.
"Clearly, the Assad regime did not get the message last year," Mr Mattis said.
