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. At least six people are reported to have been killed.
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.
It 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.
What action has the US taken?
On the orders of President Donald Trump, Navy destroyers USS Porter and USS Ross fired dozens of cruise missiles at Shayrat airfield in western Homs province at about 04:40 Syrian time (01:40 GMT).
They targeted aircraft, aircraft shelters, storage areas, ammunition supply bunkers and air defence systems at the Syrian government-controlled facility, according to the Pentagon.
It said the base was used to store chemical weapons and that "every precaution" had been taken to avoid casualties.
Speaking from his Mar-a-Lago estate in Florida, Mr Trump said he had acted in America's "vital national security interest" to prevent the use of chemical weapons.
Mr Trump branded President Assad a "dictator" who had "launched a horrible chemical weapons attack on innocent civilians".
"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.
Syrian state media said as many as nine civilians had been killed in the strike, four of them children. The BBC is unable to confirm this information.
A BBC News producer has been sharing reports from people near to the base on social media, with one describing "total devastation".
What is different about this strike?
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.
President Trump has previously spoken out against US military involvement in Syria, instead calling for a greater focus on domestic interests.
Only last week the US Ambassador to the UN, Nikki Haley, said Washington was not prioritising the removal of the Syrian president.
However, Mr Trump said "something should happen" against the Syrian leadership following the deaths in Khan Sheikhoun on Tuesday, without giving details.
US Secretary of State Rex Tillerson also signalled a sudden shift in policy on Thursday, saying that Bashar al-Assad should have no role in a future Syria.
How has Russia responded?
The Russia government is one of President Assad's most important allies and its military has been targeting all rebel groups in Syria, including jihadists such as so-called Islamic State but also the more moderate opposition forces that the US and other Western nations have been supporting.
The Pentagon said the Russian military had been informed ahead of the US action.
But the Kremlin reacted angrily to the US strike, which the Syrian army said had caused significant damage.
Dmitry Peskov, spokesman for Russian President Vladimir Putin, called it "an act of aggression against a sovereign nation".
Meanwhile, the Russian foreign ministry said it was suspending a deal with the US that was designed to prevent clashes in the skies over Syria as the two countries wage different campaigns.
The Syrian authorities accused the US of supporting terrorism by undermining the regime's operations.
What impact will the US action have? Analysis by 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.
What do we know about the 'chemical attack'?
At least 80 people died in the assault on the rebel-held town of Khan Sheikhoun in north-western Syria.
Hundreds suffered symptoms consistent with reaction to a nerve agent after what the opposition and Western powers said was a Syrian government air strike on the area.
A White House spokesman said the Trump administration believed "with a high degree of confidence" that the 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.
The Syrian military denied using any chemical agents, while its ally Russia said an air strike hit a rebel depot full of chemical munitions.
What response has there been to the US strike?
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.
Meanwhile, the UK government called the US strike "an appropriate response to the barbaric chemical weapons attack".
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.