The US has carried out a missile strike against a Syrian air base in response to a suspected chemical weapons attack on a rebel-held town.
Fifty-nine Tomahawk cruise missiles were fired from two US Navy ships in the Mediterranean. Six people were killed, the Syrian army said.
It is the first direct US military action against forces commanded by Syria’s president.
The Kremlin, which backs Bashar al-Assad, has condemned the strike.
Dmitry Peskov, spokesman for Russian President Vladimir Putin, called it “an act of aggression against a sovereign nation”.
The attack, at 04:40 Syrian time (01:40 GMT), comes just days after dozens of civilians, including many children, died in the suspected nerve gas attack in the town of Khan Sheikhoun in Idlib province.
Speaking from his Mar-a-Lago estate in Florida, Mr Trump branded President Assad a “dictator” who had “launched a horrible chemical weapons attack on innocent civilians”.
Mr Trump said he had acted in America’s “vital national security interest” to prevent the use of chemical weapons.
“Tonight I call on all civilised nations to join us in seeking to end this slaughter and bloodshed in Syria and also to end terrorism of all kinds and all types,” he said.
President Trump’s statement in full
The UK government called the US strike “an appropriate response to the barbaric chemical weapons attack”.
The Pentagon said the Russian military, which supports Syrian government forces, had been informed ahead of the US action.
In a statement the Pentagon said missiles fired from Navy destroyers USS Porter and USS Ross had targeted aircraft, aircraft shelters, storage areas, ammunition supply bunkers, air defence systems, and radars at Shayrat airfield in western Homs province.
The Pentagon added that the strike was intended “to deter the regime from using chemical weapons again”.
Analysis: Jon Sopel, BBC North America editor
Rarely has a policy changed so far and so quickly – and rarely has it been acted upon so swiftly.
When President Trump came to office the Syrian leader was seen as a useful ally in the fight against so-called Islamic State. All talk of regime change stopped.
But the chemical weapons attack changed all that. Within two days, the US has reversed its view on President Assad, identified targets and struck.
What we don’t know is whether this is a one-off act of retaliation, or the start of something more prolonged against the Assad government. Nor do we know where it leaves relations with Syria’s strong ally, Russia.
A White House spokesman said it believed “with a high degree of confidence” that Tuesday’s chemical attack had been launched from the Shayrat airfield by warplanes under the command of President Assad.
He also said the White House believed the substance used was the nerve agent Sarin, which is highly toxic and considered 20 times as deadly as cyanide.
A statement on Syrian state TV said “American aggression” had targeted a Syrian military base with “a number of missiles”.
The Syrian army spokesman later said the attack had left six people dead, a number of others wounded, and caused significant damage. He did not say whether the people affected were civilian or military.
A BBC News producer has been sharing reports from people near to the base on social media, with one describing “total devastation”.
The US has led a coalition carrying out air strikes against jihadist groups in Syria since 2014 but this is the first time it has targeted government forces.
The latest action was welcomed by Syrian opposition group the Syrian National Coalition.
“We hope for more strikes… that these are just the beginning,” spokesman Ahmad Ramadan told AFP news agency.
However, Russia said it was suspending a deal with the US designed to prevent clashes by their air forces in response to the move.
President Trump had earlier warned that “something should happen” against the Syrian leadership following the deaths in Khan Sheikhoun, but gave no details.
Also on Thursday, US Secretary of State Rex Tillerson signalled a sudden shift in policy by the Trump administration, saying that Bashar al-Assad should have no role in a future Syria.
Only last week the US Ambassador to the UN, Nikki Haley, said Washington was no longer prioritising the removal of the Syrian president.
The UN Security Council will hold further talks on Friday as it tries to agree a resolution calling for an investigation into the deaths in Khan Sheikhoun.
Russia has already rejected a Western-backed draft. Moscow has used its veto seven times to block UN resolutions critical of its ally Syria.