Moscow on Thursday called on the West to "seriously consider" the consequences of threats against Syria after the United States and France said they would respond to an alleged chemical attack.
After warning Russian Federation on Wednesday of imminent military action, U.S. President Donald Trump said on Thursday he was holding meetings on Syria and expected to make decisions "fairly soon".
Trump on Wednesday vowed to thwart Russia's missile defense system in Syria, warning that rockets "will be coming, nice and new and 'smart.'" In tweeting about a potential attack, Trump appeared to publicly telegraph military plans - something for which he heavily criticized former President Barack Obama back in 2013.
The drumbeat of military action appeared to grow louder, as Russian Federation stonewalled diplomatic efforts at the United Nations and France declared "proof" that Moscow's Syrian ally carried out a deadly chemical weapons attack that killed more than 40 Syrians.
President Donald Trump on Thursday put off a final decision on possible military strikes against Syria after tweeting earlier that they could happen "very soon or not so soon at all".
Citing US estimates that Assad has used chemical weapons "at least 50 times" in the seven-year war, Haley said: "All nations and all people will be harmed if we allow Assad to normalize the use of chemical weapons".
Syria and Russia have denied using poisonous gas in Douma on April 7, with Russian Foreign Minister Sergei Lavrov saying that Moscow had evidence that the attack in Douma was staged. Asked if France is planning to participate in retaliatory attacks on Syria, he was noncommittal.
Speaking to reporters at the White House later in the day, Trump said a decision had not yet been made on a course of action.
"You are a minority Government, and you need to seek the consent of Parliament before you commit the United Kingdom to any action".
It came nearly exactly a year after a chemical attack in the northern Syrian town of Khan Sheikhoun killed dozens of people.
But the pinpoint strike did not deter Assad and United States officials, speaking on condition of anonymity, have since investigated as many as 10 suspected chemical attacks. "Not to react is to prove to the rest of the world that what we say does not matter".
Formally, the prime minister has the right to go to war without approval from parliament, but a convention has been established in previous conflicts where MPs have a vote either before or shortly after military action begins.
But they backed action in Iraq the following year, and again in Syria in 2015, strictly limiting strikes to Islamic State (IS) group targets.
But British involvement in further military intervention is controversial at home, in a country still haunted by its role in the US-led invasion of Iraq.
Mr. Mattis said that although the United States has no hard proof, he believes the Syrian government was responsible for Saturday's attack. Multiple IS terror attacks have targeted France, including one last month.