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.
The attack at the weekend led to widespread condemnation, as Theresa May agreed with Trump and French president Emmanuel Macron that the worldwide community "needed to respond" following the suspected attack.
Britain's government weighed the possibility of military action against Syria on Thursday, agreeing the "need to take action" despite polls showing the public remains wary of military intervention.
President Donald Trump on Thursday attempted to cloud the timing of potential airstrikes on Syria, a day after indicating they were imminent.
Trump, who has often said a commander in chief should never telegraph his military intentions, apparently did so himself, tweeting that missiles "will be coming" in response to the suspected chemical attack that killed at least 40 people near Damascus.
"They agreed to keep working closely together on the worldwide response", the statement concluded.
Moscow and the Syrian regime deny that the gas attack took place or that either regime was involved.
Assad and Velayati criticised Western threats to carry out strikes on Syria in response to the alleged use of toxic weapons at the weekend, the presidency said. "Final decisions haven't been made yet on that front", Sarah Sanders said, adding that the President has not laid out a timetable.
Opposition Labour leader Jeremy Corbyn said it is "vital that parliament has the chance to debate and decide in advance" of any military action, which he warned "risks a risky escalation of the conflict".
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.
A YouGov poll in The Times conducted this week found that 43 percent of voters oppose strikes in Syria, with 34 percent unsure and only 22 percent supportive.
Twenty-one percent of the respondents said they did not know if it was necessary to hold a parliamentary vote on whether London should engage in the military actions against Syria. The country often has restricted itself to supporting roles, such as the participation of reconnaissance jets in the worldwide campaign against the Islamic State group.
"We continue to consider it extremely important to avoid any steps that could lead to more tension in Syria", Mr Peskov said.
Opposition Liberal Democrat leader Vince Cable told the BBC that parliament "can and should be recalled immediately" to hold a vote on the latest possible action.
She was one of 30 Conservative MPs to reject possible United Kingdom military action against Syria to deter the use of chemical weapons, after a vote was called by the then-Prime Minister David Cameron.
Mattis addressed a hearing of a House of Representatives committee on Thursday.