The second site targeted was a chemical weapons storage facility west of Homs, while the third was a chemical equipment storage facility and important command post.
US, British and French forces pounded Syria with more than 100 missiles early on Saturday in response to a poison gas attack that killed dozens of people last week, in the biggest intervention by Western powers against Syrian President Bashar al-Assad.
U.S., British and French officials have expressed a high degree of confidence the attack on Douma by pro-Assad forces used chlorine gas, and that it also likely used another chemical agent, possibly sarin.
Earlier, the French government said it has no samples of the chemical weapons it believes were used in Syria, but launched its military response based on open-source information and intelligence gathering.
So what happens if these strikes fail to deter Assad yet again?
As Trump deliberated, the Navy said the USS Winston S. Churchill, a destroyer armed with Tomahawk cruise missiles, was approaching the Mediterranean. The Pentagon also employed the B-1 strategic bomber. "Together, we have sent a clear message to Assad, and his murderous lieutenants, that they should not perpetrate another chemical weapons attack for which they will be held accountable".
"Nothing is certain in these kinds of matters".
Tonight, I ask all Americans to say a prayer for our noble warriors and our allies as they carry out their missions. "It was done on targets that we believed were selected to hurt the chemical weapons program. We confined it to the chemical weapons-type targets".
But other advisors want to use the opportunity to convince Trump, who wants to pull U.S. forces out of Syria once the Islamic State jihadist group is defeated, to take a tough stance.
Russia's ambassador to the United States said there would be consequences for the military strikes, adding that it was not acceptable to insult Russia's president.
Washington, Paris and London have nevertheless insisted that their own secret intelligence points to Assad's guilt, and on Friday, a United States spokeswoman said they had "proof".
Mrs May also felt there was "no graver decision" for a British Prime Minister than to have to deploy British forces as part of military action.
In a statement, French President Emmanuel Macron said, "Our response has been limited to the Syrian regimes facilities enabling the production and deployment of chemical weapons".
Announcing the military action, the US President accused President Assad of being "a monster".
"Russia in the most serious way condemns the attack on Syria where Russian military servicemen help the legitimate government to fight terrorism", Putin said.
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More than 70 people in Syria's rebel-held town of Douma reportedly died while sheltering in basements - 43 off whom showed symptoms consistent with exposure to "highly toxic chemicals", according to the World Health Organization (WHO).
Russian Federation has reacted with displeasure.
Overnight, Britain launched a joint operation with United States and French military forces to hit locations known to be involved in the production, use and storage of chemical weapons.
This means jets could strike without having to fly over Syria and avoid anti-aircraft defenses.
She said nearly a century of global acceptance about not using chemical weapons had been eroded in Douma and Salisbury.
- Four Royal Air Force Tornados contributed to the strikes, launching Storm Shadow missiles.
Defence secretary Gavin Williamson said: "The reprehensible use of chemical weapons in Douma is further evidence of the Syrian regime's appalling cruelty against its own people".
Syria's Foreign Ministry earlier said the attack coincides with the arrival of a fact-finding mission from the global chemical weapons watchdog to inspect the site of the alleged attack in the town of Douma, and "aims to hinder its work".
"We are prepared to sustain this response until the Syrian regime stops its use of prohibited chemical agents", the United States president said in a televised address.
Arizona Rep. Kirsten Sinema, a Phoenix Democrat who's running for the Senate said: "The use of chemical weapons against innocent civilians is despicable and must be condemned by every nation".
He said that Moscow called an urgent UN Security Council meeting to discuss the development.
Trump made clear in his eight-minute televised address that he was wary of a deeper entanglement in Syria, where about 2,000 USA troops have been deployed to fight Islamic State.
May ignored a convention that she seek parliamentary approval before launching any military action.
However, opposition Labour leader Jeremy Corbyn, a fervent anti-war campaigner, called the strikes "legally questionable" and said May should have recalled parliament from a holiday and "not trailed after Donald Trump". "I have done so because I judge this action to be in Britain's national interest".
"Britain should be playing a leadership role to bring about a ceasefire in the conflict, not taking instructions from Washington and putting British military personnel in harm's way", Corbyn said.
"I think it is right that the worldwide community has come together and said we will not accept this", she added.