Russian Federation has warned the West against attacking its Syrian ally President Bashar al-Assad, who is also supported by Iran, and says there is no evidence of a chemical attack in the Syrian town of Douma near Damascus.
The White House and National Security Council did not immediately respond to Anadolu Agency's requests for comment on the report.
Officials also said that the USA has compiled intelligence from the US and other countries, including images, that indicate the Syrian government was behind the weekend attack. He said he has been talking regularly this week with U.S. President Donald Trump about the most effective response.
May told her senior ministers on Thursday that the attack in Douma showed a "deeply concerning" erosion of worldwide legal norms barring the use of chemical weapons. In another tweet on Thursday, Trump wrote that an attack on Syria "could be very soon or not so soon at all!"
The U.S. officials told the American news outlet that the Assad regime is known to have used a mixture of chlorine and the nerve agent sarin in previous attacks.
Although Mr. Mattis noted that military action carried risks, he also emphasised that Syrian use of chemical weapons should not be tolerated. Asked if France is planning to participate in retaliatory attacks on Syria, he was noncommittal.
Syrian opposition activists and rescuers said poison gas was used on the rebel-held town near the capital, an allegation strongly denied by the Assad government.
The BBC said May was ready to give the go-ahead for Britain to take part in action led by the United States without seeking prior approval from parliament.
The French president does not need parliamentary permission to launch a military operation.
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". "Not to react is to prove to the rest of the world that what we say does not matter".
May and Trump had also "agreed it was vital that the use of chemical weapons did not go unchallenged, and on the need to deter the further use of chemical weapons by the Assad regime".
The Prime Minister summoned her top team to No 10 amid signs she is preparing to join US-led air strikes against Syrian targets after saying "all the indications" were that President Bashar Assad's regime was responsible for an alleged chemical attack on its own people last weekend.
The statement made no specific reference to military action.
Corbyn has said any action in Syria should be put to a parliamentary vote.
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.
France already has some 1,100 troops involved in its Operation Chammal, created in 2014 to fight Islamic State extremists in Iraq and extended in 2015 to Syria, as part of the USA -led coalition. France is already involved in the US -led coalition created in 2014 to fight the Islamic State group in Syria and Iraq.