Mattis said he would discuss Syria options at a National Security Council meeting later Thursday, and promised to keep Congressional leaders informed if the Pentagon did decide to strike.
Syria and Russian Federation have both denied the accusations, and the global chemical weapons watchdog said its team would begin its investigation on Saturday.
"We're still assessing the intelligence - ourselves and our allies".
Mattis and Joint Chiefs Chairman Marine Gen. Joseph Dunford went before the House to testify on the fiscal 2019 defense budget as Capitol Hill buzzed with speculation about President Donald Trump's threats to attack Syria and possibly start a proxy war with Russian Federation.
Meanwhile, Russian ambassador to the U.N. Vitaly Churkin said Russia does not rule out the possibility of a conflict with the USA, adding that the main priority is averting war in Syria. "We're looking for the actual evidence", Mattis told the House Armed Services Committee at a hearing Thursday.
Trump earlier this week suggested that an attack was imminent.
Shortly after his tweeted warning to Russia, Mr. Trump took a more conciliatory tone in lamenting that the U.S. -Russia relationship "is worse now than it has ever been".
The U.S., France and Britain have been in extensive consultations about launching a military strike as early as the end of this week, U.S. officials have said.
"In any event, the United States, under my Administration, has done a great job of ridding the region of ISIS", he added. Where is our "Thank you America?" "So how do we deal with this very complex situation?"
Donald Trump said at a Cabinet meeting that "major decisions on Syria" will come in the next 24 to 48 hours after he figures out who was responsible for the alleged attack in Douma - Russia, or Syria, or Iran, or "all of them together".
"We have forces in the field as you know in Syria and the use of chemical weapons in Syria is not something we should assume that, well because he didn't use them on us this time, he wouldn't use them on us next time", he said. "So they'll be made fairly soon".