In a televised address, Mr Trump said the USA was prepared to sustain its response until the Syrian regime stopped its use of chemical weapons.
She also refuted the accusation of Russian Federation in the United Nations meeting earlier on Friday that Britain was behind the Syria attack, saying Moscow tried to "completely change the story" and "turn it upside down".
May has ordered British submarines to move within missile range of Syria in readiness for strikes against the Syrian military that could begin as early as Thursday night, London's Daily Telegraph newspaper said on April 11.
Nikki Haley told an emergency meeting of the UN Security Council called by Russian Federation that should the USA and its allies decide to act in Syria it will be to defend "a bedrock global norm that benefits all nations" - the prohibition on the use of chemical weapons.
Referring to Assad, Trump said, "These are not the actions of a man".
Mr Trump warned Russian Federation on Wednesday to "get ready" for a missile attack on its ally Syria. Syrian TV reported that Syrian air defenses have responded to the U.S. -British-French attack.
Mattis said the U.S. during the Obama administration tried to deal with the chemical weapons when he was enlisting the Russians, who were complicit in the Assad regime retaining those weapons. Barzah is the location of a major Syrian scientific research center.
"We have a number of options", said White House spokeswoman Sarah Sanders, "and all of those options are still on the table".
The decision follows a suspected chemical gas attack from the Assad regime on a rebel-held town near the Syrian capital last weekend.
"We will need to take decisions in due course, when we judge it most useful and effective", Macron said.
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