The prospect of a confrontation between Russian Federation, the Syrian government's ally, and the West has loomed since Trump said on Wednesday that missiles "will be coming" in response to the attack in the Syrian town of Douma on April 7.
The council has already failed to agree on a response to the attack in three votes and has been deadlocked throughout the Syrian civil war.
President Trump says he'll make a decision "fairly soon" on whether to launch a missile attack on Syria. Then Thursday he tweeted that an attack on Syria "could be very soon or not so soon at all!"
In Paris, France's Emmanuel Macron upped the pressure on Moscow by stating he had "proof" that the Assad's regime had used chemical weapons, and vowing a response "at a time of our choosing". She told her senior ministers on Thursday the Douma events showed a "deeply concerning" erosion of global legal norms barring the use of chemical weapons.
"The chemical weapons attack that took place on Saturday in Douma in Syria was a shocking and barbaric act", May told reporters on Wednesday. The Syrian government has denied the allegations. It was not clear whether the presence of the investigators could affect the timing of any US military action.
President Donald Trump has emerged from a meeting with his national security team without a "final decision" on how to respond to a suspected chemical weapons attack in Syria. "We need to know where that's going, what the objective of it is before we take that act".
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British Prime Minister Theresa May was due to hold an emergency cabinet meeting amid speculation she will support U.S. action against the Syrian regime.
After meeting for more than two hours, the Cabinet gave May the green light to join the US and France in planning possible strikes, but also left open the possibility of other responses.
Without elaborating on the sources of her information, Ms. Haley said the American government had estimated that the forces of President Bashar al-Assad of Syria have used chemical weapons "at least 50 times" since the war started.
Opposition lawmakers have called on May to give Parliament a vote before committing British troops.
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.
Some MPs have backed Britain acting against Syria, warning that the use of chemical weapons was in breach of global law and could not be allowed to go unpunished.