The Kremlin said on Wednesday that the United States had invented reports of a chemical attack in the area, and warned against the use of this as justification for increased military action.
She said she authorized British forces to join in the strikes after intelligence indicated Syrian president Bashar al-Assad's government was responsible for an attack using chemical weapons in the Damascus suburb of Douma a week ago.
"We are prepared to sustain this response until the Syrian regime stops its use of prohibited chemical agents", the U.S. president said in a televised address.
So there is no practicable alternative to the use of force to degrade and deter the use of chemical weapons by the Syrian Regime.
She said the strikes were "right and legal" to "prevent further humanitarian crisis".
"We have seen a regime appearing to think they can use chemical weapons with impunity".
The Prime Minister explained: "I have taken this decision because I believe it is the right thing to do".
"I think it is right that the worldwide community has come together and said we will not accept this", she added.
Dozens of civilians were killed in the chemical weapons attack in Douma.
US President Donald Trump addresses the nation on the situation in Syria April 13, 2018 at the White House in Washington, DC.
In her comments, May also alluded to a nerve agent attack in Britain last month on a former Russian spy and his daughter.
Besides Trump, French President Emanual Macron said in a statement that they also could not tolerate the normalisation of the use of chemical weapons.
British Prime Minister Theresa May described the strike as "limited and targeted".
May is not obliged to win parliament's approval before ordering military action, but a non-binding constitutional convention to do so has been established since a 2003 vote on joining the USA -led invasion of Iraq.
"We have. evidence that proves Britain was directly involved in organising this provocation", defence ministry spokesman Igor Konashenkov said.
For his part, the leader of the UK Independence Party (UKIP), Gerard Batten, opposed the British military action in Syria.
Prime Minister Justin Trudeau said in a statement that Canada stands with its allies and that it supports the decision " to take action to degrade the Assad regime's ability to launch chemical weapons attacks against its own people".
The UK has begun air strikes against Syrian chemical weapons sites.
Mrs May said chemical weapons had "all too often" been used in recent times.