But asked what is the best approach to deal with the conflict in Syria, Mattis said the USA remains committed to "ending that war through the Geneva process, the United Nations orchestrated effort".
The action Friday was the second such strike Trump has authorized in Syria after the USA launched 59 Tomahawk missiles from warships at a Syrian airbase in April 2017.
The Pentagon is now working under war powers granted in the aftermath of the September 11, 2001 attacks, even though operations across the Middle East have morphed broadly since those early days of seeking to destroy Al-Qaeda.
"I am confident the Syrian regime conducted a chemical attack on innocent people", Mattis said.
Macron said France has proof that the Syrian government launched chlorine gas attacks and said France would not tolerate "regimes that think everything is permitted". He is not giving details about what equipment is involved in the operation or what sites it is targeting.
Trump said he would make a decision "fairly soon" about a strike.
He says increased engagement from countries including Saudi Arabia, the United Arab Emirates, Qatar and Egypt can ensure that Iran does not profit from the defeat of the Islamic State group. In his conversation with May, the two "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", according to May's office.
Mattis said that any further strikes would "depend on Mr. Assad, should he decide to use more chemical weapons in the future".
The United States launched cruise missiles at Assad's armed forces past year after a similar gas attack.
He said that they were still trying to get Organization for the Prevention of Chemical Weapons inspectors to Douma within the week to collect evidence.
Watch Trump announcing the strikes here.
And he described the Middle East as a "troubled place".
If Tomahawk missiles were used, the minimum weapons cost of the Friday strike would be $165 million.
Speaking before a meeting with farm state lawmakers on Thursday morning, Trump said he and his team were closely monitoring events on the ground.
Trump spoke from the White House Friday night. "So how do we deal with this very complex situation", Mattis said. "Protection of our forces, I don't think we need to wait until they are under chemical attack when the weapons are used in the same theater we are operating in". Trump wanted Mattis "to push the limits a little bit more", a White House official told the Wall Street Journal, while Mattis was said to be reluctant. "He's moreover going to hold the Russians and the Iranians who're propping up this regime accountable".
Trump's United Nations ambassador, Nikki Haley, talked about the president had not however made a final willpower, two days after he tweeted that Russian Federation must "put together" on account of a missile assault "is likely to be coming" at Moscow's chief Middle East ally.
Mattis is also concerned a strike against Syria could set off a confrontation with Russian Federation.