Defense Secretary Jim Mattis signaled Thursday that the U.S. was prepared to wait for definitive proof that Syria carried out a chemical attack before launching an attack, but stressed the moral imperative for a U.S. response.

"Some things are simply inexcusable, beyond the pale and in the worst interest of, not just the chemical weapons convention, but of civilization itself," Mattis said in testimony before the House Armed Services Committee.

Currently, the U.S. and its allies "don't have evidence" that the Syrian regime carried out the attack last Saturday in the Damascus suburb of Douma that reportedly killed at least 40, Mattis said.

"I believe there was a chemical attack and we're looking for the evidence," he said. "We're trying to get inspectors in, probably within a week."

Mattis was referring to international investigating teams from the Organization for the Prohibition of Chemical Weapons (OPCW).

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.

"We will not know from this investigating team" who was responsible, Mattis said, although he theorized that chlorine or sarin nerve agent -- or a combination of the two -- were used in the barrel bombing of Douma.

"We will not know who did it. Only that it happened. That's where we're at right now," Mattis told the committee.

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 Russia.

Trump earlier this week suggested that an attack was imminent.

"Get ready Russia," he said in a tweet.

But he suggested in another tweet early Thursday that there was no timeline for a U.S. response.

"Never said when an attack on Syria would take place," Trump said. "Could be very soon or not so soon at all! In any event, the United States, under my Administration, has done a great job of ridding the region of ISIS. Where is our "Thank you America?"

Following an agriculture meeting at the White House later Thursday, Trump said of Syria that "we'll see what happens," but he expected decisions would be made "fairly soon."

He said "We're looking very seriously at that situation. It's too bad the world puts us in a position like that." He continued that decisions will be "made fairly soon."

Following their testimony before the Committee, Mattis said that he and Dunford were going to the White House for a National Security Council meeting at which he will "take forward the various options to the president."

-- Richard Sisk can be reached at Richard.Sisk@Military.com.