Washington (CNN) President Donald Trump continued to weigh his options Thursday for responding to a chemical gas attack in Syria as his aides warned a miscalculated response could spark a dramatic escalation between the United States, Russia and Iran.

Military brass and senior national security officials huddled at the White House for 90 minutes on Thursday afternoon to consider their options. But afterward the White House said that "no final decision has been made."

Earlier Thursday, Trump appeared to soften his rhetoric about imminent airstrikes on Syria, a day after warning Russia that missiles "will be coming, nice and new and 'smart.'" In an early morning tweet and later in comments at the White House, Trump worked to cloud the timing of military action and said a final decision had not yet been made.

"Never said when an attack on Syria would take place. Could be very soon or not so soon at all!," the President tweeted.

The ongoing deliberations reflected the conundrum Trump faces as he contemplates wading further into Syria's multi-sided civil war. He has pressed his aides and US allies to develop a more muscular response to the weekend chemical attack after strikes last year did little to prevent the Syrian regime from continuing their assault on civilians.

But his military advisers have warned that a scaled-up response risks drawing the United States into further conflict with Russia and Iran, which support Syrian dictator Bashar al-Assad.

"We are 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," Mattis said in morning testimony before the House Armed Services Committee.

Mattis was expected to voice those concerns in Thursday's meeting at the White House, according to US officials.

Some lawmakers, meanwhile, are warning the administration that military action in Syria would require congressional authorization, though House Speaker Paul Ryan downplayed those concerns during a news conference.

Speaking before a meeting with farm state lawmakers on Thursday morning, Trump said he and his team were closely monitoring events on the ground.

"We're looking very, very seriously, very closely, at that whole situation," the President said from the Cabinet Room. "We have to make some further decisions. So they'll be made fairly soon."

Earlier in the week, Trump suggested he was committed to ordering strikes in Syria. He said on Monday that a decision would be made within 48 hours. On Wednesday he vowed to thwart Russia's missile defense system in Syria, warning that rockets "will be coming, nice and new and 'smart.'"

But since then he's suggested he'll wait for further advice from military advisers and US allies. The White House said he was planning to speak with French President Emmanuel Macron and British Prime Minister Theresa May on Thursday evening.

Trump has remained in heavy consultation with France and the United Kingdom since the suspected chemical attack, which killed more than 40 people. Officials said Thursday the trio has yet to reach firm agreement on the scale or timing of a response. Another US ally, Germany, said on Thursday they would not participate in military action in Syria.

Should the President follow through on his warnings of an attack, two US Navy destroyers armed with Tomahawk cruise missiles are in position and ready to be called into action, among other assets, including jets and submarines.

Macron and May both suggested on Thursday that enough evidence had been gathered to pin blame for the attack on Assad.

"It is highly likely that the regime is responsible for Saturday's attack," a Downing Street spokesman said in a readout of a Cabinet meeting that May convened Thursday evening in London.

Macron went further in an interview with France's TF1, suggesting the gas used was chlorine.

"We have proof that last week ... chemical weapons were used, at least chlorine was, and they were used by the Assad regime," he said.

Those comments go further than US officials have been willing to in public. Mattis told lawmakers he hasn't seen firm proof that Assad's regime was behind the attack, though added that he believed it was.

"We are continuing to assess intelligence and are engaged in conversations with our partners and allies," White House press secretary Sarah Sanders said in a statement on Thursday afternoon.

International observers with the Organization for the Prohibition of Chemical Weapons have yet to reach the Damascus suburb of Douma, where the attack took place. The organization said they were due to arrive on Saturday.

In warning against a missile strike, Russian Foreign Ministry spokeswoman Maria Zakharova said on Wednesday that such a move could destroy evidence on the ground and interrupt the work of international investigators.

"Smart missiles should fly toward terrorists, not the legal government that has been fighting international terrorism for several years on its territory," Zakharova wrote in response to Trump's Wednesday tweet.