WASHINGTON — President Donald Trump said Thursday a decision on U.S. action in Syria would come “very soon or not so soon,” signaling a slowing of what had seemed a quick drive for airstrikes in retaliation for the suspected use of chemical weapons against Syrian civilians.

Defense Secretary Jim Mattis also raised caution flags Thursday, musing aloud about the risks of an escalating war, even as he told Congress that the Pentagon would present options for a Syria response at a National Security Council meeting Thursday.

“Never said when an attack on Syria would take place. Could be very soon or not so soon at all!” Trump wrote on Twitter.

That walked back a Trump tweet Wednesday that announced strikes “are coming,” and warned Syrian ally Russia against trying to shoot the U.S. missiles down.

That taunt took allies and administration officials by surprise, and alarmed some military officials who are advocating a deliberate approach that draws in allies and presents a clear case for why U.S. action is warranted, U.S. officials said on condition of anonymity because the internal discussions are continuing.

Mattis seemed to acknowledge those qualms, however. With Russia and Iran heavily invested in Syrian President Bashar Assad’s survival, Mattis suggested the Pentagon would advise caution in discussing possible actions with the president.

“We’re trying to stop the murder of innocent people, but, on a strategic level, it’s how do we keep this from escalating out of control, if you get my drift on that,” the retired four-star Marine general told the House Armed Services Committee.

White House press secretary Sarah Huckabee Sanders issued a statement late Thursday, following the meeting with Mattis and national security advisers.

“No final decision has been made,” on Syria, Sanders said. “We are continuing to assess intelligence and are engaged in conversations with our partners and allies,” Sanders said, adding that Trump planned to speak with French President Emmanuel Macron and British Prime Minister Theresa May later in the evening.

Separately, a senior U.S. official said top Pentagon brass have argued that quick military action may have unintended consequences, including with Russia. Officials further contended that Trump could look weak if — like a unilateral military strike on Syria a year ago — a new assault failed to deter Assad.

Missile strikes have appeared likely since the deaths of families from what the U.S. has called a poison gas attack on the rebel-held town of Douma, near Damascus.

Trump’s options include the sort of limited response he ordered last year in response to another suspected use of chemical weapons or a heavier assault designed to show the cost Assad brought on himself by using such weapons again, said one official familiar with military and diplomatic discussions on Syria.

Macron appeared to take a step Thursday toward joining the United States in a forthcoming attack, claiming that France has “proof” of a chemical attack and insisting anyone who commits such abuses be held to account.

Macron’s comments were widely interpreted as an argument directed at critics worried about a reprise of France’s participation in a 2011 NATO intervention in Libya, which helped bring down ruler Moammar Gadhafi but threw Libya into deeper chaos.

On Monday, Trump had said a decision on a U.S. response to the weekend deaths of more than 40 civilians would come within 48 hours. That time frame elapsed with no explanation from the White House.

“Now we have to make some further decisions,” Trump said during a brief appearance before reporters at the White House on Thursday afternoon. “They’ll be made fairly soon.”

“We’ll see what happens,” Trump said. “We’re obviously looking at that very closely. ... It’s too bad that the world puts us in a position like that.”

The debate about when, or whether, to strike in Syria follows Trump’s surprise promise earlier this month to withdraw U.S. forces from Syria “very soon.” Other nations should step in, Trump said. Like his remarks about the timing of strikes, that comment startled and alarmed military officials who argued that the U.S. counterterrorism mission has not run its course.

Trump’s initial tweet Thursday about timing included a complaint about the military burden shouldered by the United States.

“In any event, the United States, under my Administration, has done a great job of ridding the region of ISIS,” Trump wrote. “Where is our “Thank you America?”?”

It also conflated international outrage over the alleged use of chemical weapons with the counterterrorism mission in Syria, a fight on which the United States, Russia and Assad are on the same side.

Mattis said the United States had yet to obtain hard evidence linked to the attack.