Shortly after Trump began a White House address to announce the action, large explosions were heard in the Syrian capital Damascus, signalling a new chapter in a brutal seven-year-old civil war.

"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 speech to the nation.

"A combined operation with the armed forces of France and the United Kingdom is now under way. We thank them both. This massacre was a significant escalation in a pattern of chemical weapons use by that very terrible regime."

A US official said the strikes had targeted chemical production facilities. Another official said multiple types of bombs were used, and a variety of targets chosen.