"We assessed that this was the primary location of Syrian sarin and precursor production equipment", he said.
Mattis did not reveal the number of missiles used in the strike, but said the USA and its partners had "used a little over double the number of weapons this year than we used last year".
White House spokesman Raj Shah talked about Friday afternoon that Trump "goes to hold the Syrian authorities accountable".
Asked about concerns in planning a strike on Assad's regime, Mattis cited worry over inadvertently causing civilian casualties and avoiding anything that would trigger a wider war.
Russia's U.S. Ambassador said the attack "will not be left without consequences". The next Syrian attack - strike three - will likely produce a large-scale USA response. Defense Department officials said that will include trying to get more commitments from allies of help immediately after any strikes. Jim Inhofe, an Oklahoma Republican, said the USA should "go over and bomb".
Mattis said that although the United States has no hard proof, he believes the Syrian government was responsible for Saturday's attack.
"Good souls will not be humiliated", Syria's presidency tweeted after airstrikes began.
Mattis said the United States has yet to confirm details of the attack through worldwide investigators.
Syria and Russian Federation have both denied the accusations, and the global chemical weapons watchdog said its team would begin its investigation on Saturday.
Trump calls the two countries those "most responsible for supporting, equipping and financing the criminal Assad regime".
Messages are being passed to Moscow about the U.S. and its allies' intentions to create a "lasting deterrent" against the use of chemical weapons again, according to one senior administration official. "First of all, we are committed to ending that war through the Geneva process through the United Nations -orchestrated effort". But shortly afterward the White House said "no final decision has been made".
Speaking from the White House, Trump says, "To Iran and to Russian Federation, I ask: What kind of a nation wants to be associated with the mass murder of innocent men, women and children?"
To quote The Dude from "The Big Lebowski," the widely expected military strikes on Syria are a complicated matter with "a lot of ins, a lot of outs, a lot of 'what have yous, ' and a lot of strands to keep in my head, man".
"We will not know who did it".
However, Rep. Adam Smith, D-Wash., ranking Democrat on the committee, warned a soaring deficit is facing the United States as a result of an expanding budget and the military must prepare for tough questions on spending in the future.
The U.S. acted together with the French and the British.