Defense Secretary James Mattis testifies before the House Armed Services Committee on April 12, 2018. (Photo: Screen grab/C-SPAN)

(CNSNews.com) - "We have not yet made any decision to launch military attacks into Syria," Defense Secretary James Mattis told the House Armed Services Committee on Thursday. He also said a military response is only one option. The others are diplomacy and economic measures.

President Trump on Wednesday tweeted: "Russia vows to shoot down any and all missiles fired at Syria. Get ready Russia, because they will be coming, nice and new and 'smart!' You shouldn’t be partners with a Gas Killing Animal who kills his people and enjoys it!"

But by Thursday, Trump backed off a bit, tweeting: "Never said when an attack on Syria would take place. Could be very soon or not so soon at all!"

Mattis on Thursday was responding to a question from Rep. Tulsi Gabbard (D-Hawaii), who noted that Syria has not threatened or declared war against the U.S., and therefore an attack by the U.S. would violate the U.S. Constitution, which gives the president the power only to repel sudden attacks, not to declare war.

Gabbard also noted that an attack on Syria ordered by Trump would violate the War Powers Resolution, which requires the president to consult with Congress before sending troops into hostilities. And a Trump-ordered attack would violate the omnibus spending bill, signed by Trump this year, that says none of the funds made available by the act may be used in Syria in contravention of the War Powers Resolution.

Mattis replied that President Barack Obama faced a similar situation when he sent troops into Syria to go after a terrorist group, ISIS, which is not named in the 2001 Authorization for the Use of Military Force.

"This is a complex area, I'll be the first to admit," Mattis said.

Gabbard pressed Mattis on whether Trump will comply with the Constitution and the law.

"I believe that the president will carry out his duties under the Constitution to protect the country," Mattis replied.

Asked how an attack on Syria would serve the interests of the American people, Mattis demurred: "I don't want to talk about a specific attack that is not in offing," he said.

"Again, the president has not made that decision. However, looking at the chemical warfare convention, I thinks it's by far in the best interest of civilization, certainly in the best interest of America, that that convention be obeyed by the nations that have signed it," Mattis said.

"And what has happened in Salisbury, England and now has happened in Syria -- again shows that this is not an idle concern."

Mattis, asked risking a war with Russia over Syria serves American interests, said he's "not ready to speculate that that would happen."

"And there's a lot of ways to respond to the ...violation of the chemical weapons convention -- diplomatically, economically, militarily -- taken in total -- would represent, I think, what we have to do in this world if we're going to turn it over in accord with international norms, international law."