"We are not going to engage in the civil war itself".
"It's got to end, and our strategy remains the same as a year ago. It is to drive this to a United Nations brokered peace but at the same time keep our foot on the neck of ISIS until we suffocate it".
Later on Thursday he was noncommittal.
Trump said that same day from the Cabinet Room, "We're looking very, very seriously, very closely, at that whole situation".
The US, France, and the United Kingdom, appear on the verge of combat with Syria, and possibly its ally Russian Federation. "We're still working on this", Mattis told reporters at the Pentagon prior to meeting with defense officials from the Netherlands.
Mattis' reservations are in stark contrast to newly appointed national security adviser John Bolton, a noted foreign policy hawk who favors an attack that would be "ruinous" for the Syrian regime, the Journal reports.
President Emmanuel Macron of France said on Thursday that French authorities had proof that the Syrian regime had launched the chemical attack.
He cautioned that even if the inspectors get access to the attack sites, they are not permitted under their mandate to assign blame, but only to determine that chemical agents were used.
Mattis said that although the United States has no hard proof, he believes the Syrian government was responsible for Saturday's attack. "Thereby, the use of sarin and chlorine is not confirmed". It was not clear whether the presence of the investigators could affect the timing of any U.S. military action.
Mattis, addressing a hearing of the House of Representatives Armed Services Committee, declined to discuss USA military planning on Syria.
The U.S. and its allies are "still assessing the intelligence" needed to prove the Assad regime conducted a recent alleged chemical attack in Syria, U.S. Defense Secretary James Mattis said Wednesday.
Asked what his biggest worry is about potential military action, Mattis cited civilian casualties. "But on a strategic level, it's (about) how do we keep this from escalating out of control", Mattis said. And we'll see what happens, folks. More broadly, he doubted the wisdom of bombing.
"I hope you are also planning for a lean future", Smith said. "We need to know where that's going, what the goal of it is before we take that act".
Any aggressive U.S. military action in Syria could lead to escalation and broader conflict with Moscow and Tehran, which both have a heavy military presence in the Arab nation. Jeffrey also said that he thinks Mattis is anxious about losing focus on North Korea, which could result from an attack Syria.
But Democratic Congresswoman Tulsi Gabbard disagreed, and said last year's strike on a Syrian airfield following a chemical attack in Khan Sheikhun was illegal.