US President Donald Trump on Friday ordered precision strikes targeting Syrian President Bashar al-Assad's chemical weapons capabilities after a poison gas attack that killed at least 60 people on April 7.
Trump said a combined operation with France and Britain was under way.
"A short time ago, I ordered the United States Armed Forces to launch precision strikes on targets associated with the chemical weapons capabilities of Syrian dictator Bashar al-Assad," Trump said in a televised address from the White House.
"These are not the actions of a man. They are crimes of a monster instead," Trump said referring to Assad and his role in the chemical weapons attacks.
As he spoke, explosions were reported in Damascus.
"The purpose of our actions tonight is to establish a strong deterrent against the production, spread and use of chemical weapons," Trump said.
British Prime Minister Theresa May affirmed that her country's forces had joined the attack. "We cannot allow the use of chemical weapons to become normalized — within Syria, on the streets of the UK, or anywhere else in our world."
The United States is using Tomahawk cruise missiles in its strikes in Syria, and taking aim at multiple targets in the country, a US official told Reuters on Friday, without disclosing whether US aircraft were also involved in strikes.
At least six loud explosions were heard in Damascus on Saturday and smoke was seen rising over the Syrian capital, a Reuters witness said.
Syrian state TV said the army's air defenses were confronting an attack by the United States, France and Britain.
A second witness said the Barzah district of Damascus had been hit in the strikes. Barzah is the location of a major Syrian scientific research center.