Updated at 10:46 p.m. ET
President Trump announced Friday night that the U.S. and its allies had launched attacks on Syria in response to an alleged chemical weapons attack last week by President Bashar Assad's regime.
In televised remarks from the White House, Trump said the attacks were underway and that Great Britain and France were also taking part.
"We are prepared to sustain this response until the Syrian regime stops its use of prohibited chemical agents," the president said.
The president did not provide details of the military action, but the Pentagon said the U.S., British and French naval and air forces struck three sites linked to the country's chemical weapons program.
News agencies reported large explosions in the capital, Damascus, but there was no immediate word on any casualties or damage.
About an hour after the president spoke, Defense Secretary Jim Mattis said the airstrikes were over.
"Right now we have no additional attacks planned," Mattis said.
Gen. Joe Dunford, the chairman of the Joint Chiefs of Staff, said the U.S. used its regular communication channels with the Russians to tell them in advance that U.S. aircraft would be in the region. But the U.S. did not warn the Russians about the targets, he added.
The U.S. has about 2,000 troops in northeastern Syria, where they have partnered with Kurdish fighters against the Islamic State.
The president again said that he supported the withdrawal of U.S. forces.
"America does not seek an indefinite presence in Syria," he said. "The United States will be a partner and a friend. But the fate of the region lies in the hands of its own people."
Trump also addressed the governments of Iran and Russia: "I ask what kind of a nation wants to be associated with the mass murder of innocent men, women and children. The nations of the world can be judged by the friends they keep. No nation can succeed in the long run by promoting rogue states, brutal tyrants and murderous dictators."
The suspected chemical attack occurred on April 7 in the Damascus suburb of Douma, which had been held by rebel forces for years in Syria's continuing civil war. Civilians and rebel forces abandoned Douma following the latest attack.
"This massacre was a significant escalation in a pattern of chemical weapons use by that very terrible regime," the president said of the April 7 incident.
Referring to Assad, Trump said, "These are not the actions of a man. They are crimes of a monster instead."
In the days leading up to the U.S. attack, Russia had warned that it would defend its troops in Syria. This has raised fears of a possible direct clash of U.S. and Russian forces.
The U.S. strike comes almost exactly a year after a previous U.S. strike, which was also in response to the Syrian regime's alleged use of chemical weapons. In that strike, on April 7, 2017, the U.S. launched 59 cruise missiles on a Syrian military airbase.
However, over the past year, Assad's regime has improved its position in much of the country as it battles various rebel groups. His government controls most of the major cities, which are located in the western part of the country.