On Thursday, White House Press Secretary Sarah Sanders said Mr Trump would continue to assess intelligence and engage with allies.
Britain would work with the United States and France to coordinate an worldwide response, it added.
Russian Federation is a major backer of the government of Syrian President Bashar al-Assad in the country's civil war, and strongly opposes the US claim that Syrian government forces attacked a rebel-held town near Damascus on Saturday with chemical weapons. Trump tweeted Wednesday that missiles "will be coming" and on Thursday tweeted that an attack "could be very soon or not so soon at all!" Trump notably did not rule out plans to attack Syria in retaliation for the weekend's suspected chemical attack on civilians at the hands of the Assad regime.
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 unsafe escalation of the conflict".
Macron doesn't need parliamentary permission to launch an operation. The Syrian government has denied the allegations.
According to the statement, May described the chemical attack on Douma in Eastern Ghouta as a "shocking and barbaric act which killed up to 75 people, including children, in the most appalling and inhumane way".
Asked about the risks of US military retaliation, Mattis cited two concerns, starting with avoiding civilian casualties.
Mr. Mattis said he personally believes Syria is guilty of an "inexcusable" use of chemical weapons, while noting that the global fact-finding team would likely fall short of determining who was responsible. "We need to know where that's going, what the objective of it is before we take that act".
US President Donald Trump is consulting with British and French leaders as he mulls how to respond to an alleged chemical weapons attack in Syria.
May is not obliged to win parliament's approval, but a non-binding constitutional convention to do so has been established since a 2003 vote on joining the USA -led invasion of Iraq.
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.
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.
Britain has launched air strikes against Islamic State militants in Syria, but not against the country's government.
It has been observed in subsequent military deployments in Libya and Iraq and many British lawmakers and voters are deeply sceptical of deepening involvement in the Syrian civil war.
The attack in Douma happened late on Saturday amid a resumed offensive by Syrian government forces after the collapse of a truce with the Army of Islam rebel group. Multiple IS terror attacks have targeted France, including one last month.