The US has carried out a missile attack against targets in Syria in response to a suspected chemical weapons attack on a rebel-held town.
A Pentagon official said about 50 Tomahawk cruise missiles were fired from US Navy destroyers in the eastern Mediterranean at a Syrian air base.
In a televised address, President Donald Trump said the base was where Tuesday's attack was launched from.
He called on "all civilised nations" to help end the conflict in Syria.
Dozens of civilians, including many children, died in the suspected nerve gas attack on the town of Khan Sheikhoun in Idlib province.
Speaking from his Mar-a-Lago estate in Florida, President Trump branded Syrian President Bashar al-Assad a "dictator" who had "launched a horrible chemical weapons attack on innocent civilians".
"Tonight I call on all civilized 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.
"We hope that as long as America stands for justice, then peace and harmony will in the end prevail."
The Pentagon said Russia, which is supporting the Syrian military, had been informed ahead of the US attack.
A White House spokesman said the site targeted by the cruise missiles "was directly linked to the horrific chemical weapons attack".
"We assess with a high degree of confidence that the chemical weapons attack earlier this week was launched from this site by air assets under the command of the Assad regime," the official said.
"We also assess, with a similar degree of confidence, that the Assad regime used a chemical nerve agent consistent with Sarin in these attacks."
A statement on Syrian state TV said "American aggression" had targeted a Syrian military base with "a number of missiles" but gave no further details.
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.