He said the United States is considering a response, including military attacks. He also warned Russians to make sure no troops were in the area.
The US, France and Britain have been consulting about launching a military strike. On the same day, OPCW Director-General Ahmet Uzumcu said the organization had made a decision to send its experts to Syria.
The Massachusetts Democrat said that while Assad's barbaric use of chemical weapons must never again occur, Trump "must come to Congress for authorization" prior to another military operation against Syria.
During a Cabinet meeting Thursday morning, the president said, "We are looking very seriously, very closely at that situation and we will see what happens folks".
Russia, a Syrian military ally, has called for restraint, saying there was no evidence that the Syrian government used a banned chemical agent in Douma.
The Syrian government said it would facilitate the mission's investigation, which is to start on Saturday.
Macron's office and the French military aren't commenting on pending plans. France is already involved in the US -led coalition created in 2014 to fight the Islamic State group in Syria and Iraq. Multiple IS terror attacks have targeted France, including one last month.
Over the weekend, Mr. Trump blamed Syrian President Bashar Assad and his worldwide allies, including Russian President Vladmir Putin for the apparent attack that left dozens dead and hundreds injured. Officials suggested such evidence was lacking, or at least not yet at hand.
Officials and analysts in Washington say a robust, US -led retaliatory military response is expected within days.
However, he also emphasized that Syrian use of chemical weapons should not be tolerated.
Mr Assad said Western countries were lashing out after they lost their "bet" on opposition forces in the eastern Ghouta suburbs of the Syrian capital.
"We're considering all of those options and a number of different timetables of what any action we may or may not take would look like", Sanders said.
However the U.S. broadcaster NBC later quoted United States officials familiar with the intelligence as saying they had now obtained blood and urine samples which had tested positive for chemical weapons.
The White House said on Thursday (Friday NZT), that Trump would consult further with allies.
Mr Trump has spoken to British Prime Minister Theresa May and the two leaders talked about the "need for a joint response to Syria's use of chemical weapons", the White House said.
Mattis spoke before a House Armed Services Committee hearing a day after Trump taunted Russian Federation, warning of an attack in Syria.
The attack is the latest in a series of aggressive bombings in Douma, where Russian-backed forces have been working to eradicate Syrian rebels. It was not clear whether the presence of the investigators could affect the timing of any U.S. military action.
The suspected use of chemical weapons has drawn the West back into Syria's seven-year conflict.