Donald Trump has announced he ordered a cruise missile strike on a Syrian air base from where a devastating chemical attack on civilians was launched, saying there could be "no dispute" Bashar Assad was to blame.
The surprise barrage of around 60 cruise missiles in the early hours of Friday, UK time, was the first direct US attack on the Syrian government.
Speaking from Florida, Mr Trump said he was reacting to Syrian president Assad's use of a nerve agent, in an emotional message to the public in which he evoked images of children dying.
The US president said: "Using a deadly nerve agent, Assad choked out the lives of helpless men, women and children. It was a slow and brutal death for so many.
"Even beautiful babies were cruelly murdered in this very barbaric attack.
"Tonight I ordered a targeted military strike on the airfield in Syria from where the chemical attack was launched.
"It is in this vital national security interest of the United States to prevent and deter the spread and use of deadly chemical weapons.
"There can be no dispute that Syria used banned chemical weapons."
The Tomahawk missiles, launched from two warships in the Mediterranean Sea, targeted airstrips, hangars, control towers and ammunition areas in Sharyat, central Syria, according to officials.
At least 72 people, including 20 children, were killed by a suspected mixture of chlorine and a nerve agent in an attack in the largely opposition-held Idlib province on Tuesday.
Mr Trump said the latest action was in the "vital national security interest", adding that the US must "prevent and deter the spread and use of deadly chemical weapons".
He also called for other "civilised nations" to join efforts "seeking to end the slaughter and bloodshed in Syria".
Britain had been leading renewed calls for diplomatic action in response to the earlier chemical attack.
The US, UK and France had brought a resolution before the United Nations Security Council, demanding an investigation.
Earlier on Wednesday, Downing Street had played down the prospect of military action, insisting "nobody is talking" about an armed response to the atrocity.
Syrian state TV went on to report missile attacks on a number of military targets, calling them an act of "aggression".
Pentagon spokesman Captain Jeff Davis said the strike was a "proportional response to Assad's heinous act".
It succeeded in "reducing the Syrian government's ability to deliver chemical weapons" by severely damaging or destroying aircraft, according to initial indications, he added.
Secretary of state Rex Tillerson said Russia had "failed" to deliver its commitment to secure Syria's chemical weapons, saying it had been either complicit or "simply incompetent".