The President will meet with his national security team at the White House on Thursday for further discussions on the United States response.
Prime Minister Theresa May was holding an emergency cabinet to discuss joining mooted strikes by the United States and allies, as rival politicians and some Conservative colleagues called for a parliamentary vote before any British involvement.
Outlining the reasons why he voted against, he said: "One was because he (Mr Cameron) hadn't provided the evidence and intelligence that we knew who it was, and secondly because there was not a proper plan which was thought through properly".
Following the meeting, May spoke to Donald Trump and the pair agreed that the United Kingdom and the USA would "keep working closely together on the global response", according to a statement from Downing Street.
Trump had vowed to take swift and decisive action against Syria since reports of a suspected poison gas attack on a rebel enclave that killed dozens of people.
French President Emmanuel Macron on Thursday said France has proof that the Syrian government launched chlorine gas attacks and that France would not tolerate "regimes that think everything is permitted". France is already involved in the US -led coalition created in 2014 to fight the Islamic State group in Syria and Iraq.
Syria's President, Bashar al Assad, said a possible attack on his country could trigger regional instability.
But rival politicians and some Conservative colleagues have called for a parliamentary vote before any British involvement.
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Syria warned that it will have "no other choice" but to defend itself if the West launches military action. A YouGov poll showed just one in five members of the public support a strike on 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 US -led invasion of Iraq.
But they backed action in Iraq the following year, and again in Syria in 2015, strictly limiting strikes to Islamic State (IS) group targets.
British lawmakers voted down taking military action against Damascus in 2013, in what was widely viewed as an assertion of parliamentary sovereignty on the use of force.
Britain continues to support the US-led coalition targeting IS jihadists in Iraq and Syria, and has conducted more than 1,700 strikes.
"We're looking very, very seriously, very closely, at that whole situation", the President said during a meeting with farm state lawmakers.
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. 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.
Speaking to congress at a hearing of the House Armed Services Committee on Thursday, Mattis said he believed a chemical attack had taken place and blamed Russian Federation for being complicit in Syrian government's possession of chemical weapons.