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.

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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 said.

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, including children, from what the United States 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.

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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 deployment 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 timeframe 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 at the White House. "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.

"I believe there was a chemical attack, and we're looking for the actual evidence," he said in testimony before the House Armed Services Committee.

In a statement, Maj. Adrian Rankin-Galloway, a Defense Dept. spokesman, said the victims of the Douma attack had symptoms "reported by credible medical professionals and visible in social media photos and video [that] are consistent with an asphyxiation agent and of a nerve agent of some type."

Mattis said he hoped that experts from the Organization for the Prohibition of Chemical Weapons (OPCW) would soon be granted access to the affected areas, but noted that inspectors would not have the ability to determine responsibility for the attack.

International legitimacy for a U.S. or allied strike would rest largely on such findings. Syria and Russia deny that Assad's forced deployed chemical agents. Russia argued at the United Nations this week that if such weapons were used it was an attack carried out by anti-Assad rebels.

The Netherlands-based OPCW said Thursday that a fact-finding mission is en route to Syria and will start investigating the suspected chemical attack on Saturday. The same organization has announced findings linking Russia to a suspected nerve agent attack on a former Russian spy in Britain last month.

Asked about the legal basis for a potential action in Syria, Mattis suggested it could be framed as a self-defense strike given the proximity of a force of about 2,000 U.S. troops.

"The use of chemical weapons in Syria is not something we should assume ... 'Well, because he didn't use them on us this time, he wouldn't use them on us next time,'" he said.

British Foreign Secretary Boris Johnson appeared to cast doubt Thursday on an early strike, saying that Britain is calling for a meeting of the OPCW next week "to discuss next steps."

"We will now work tirelessly with our partners to help stamp out the grotesque use of weapons of this kind," Johnson said. "The Kremlin must give answers."

German Chancellor Angela Merkel said Thursday that her country would not join a military operation in Syria.

Critics called Trump a hypocrite for his tweets on Wednesday, which previewed military action.Before and during the 2016 presidential campaign, Trump called President Barack Obama foolish and weak for announcing strategy or deadlines for military action.

In April 2017, as he contemplated a strike in Syria, Trump said, "One of the things I think you've noticed about me is: Militarily, I don't like to say where I'm going and what I'm doing."

Asked during his weekly news conference if he thought Trump was "dithering," House Speaker Paul Ryan, R-Wis., defended the president, saying he was right to consult with allies about a unified response.

"I think you're wrong to suggest he's dithering," Ryan said. "He's not dithering."

Ryan said that he thinks "it's important for us to help lead the international community to make sure people are held accountable for these mass atrocities."