(CNN) As someone who once criticized President Barack Obama for offering public timelines on American military engagement overseas, US President Donald Trump may have backed himself into a corner following his own promise to respond, and respond harshly, to the deadly chemical attacks in Syria.

Last week he declared he wanted to withdraw US troops from Syria as soon as possible. But the suspected use of chemical weapons in Syria over the weekend may force him to reevaluate the US mission in that country.

And those words have kept coming. Trump tweeted threats of using "new and smart" missiles against Syria on Wednesday , directing hostility towards Russia in the same breath.

In the days since the alleged attack:wi

Trump has canceled his trip to South America where he was meant to attend the 8th Summit of the Americas. "The President will remain in the States to oversee the American response to Syria and to monitor developments around the world."

Defense Secretary James Mattis has rearranged his schedule, canceling trips to California and Nevada and keeping his activities for the weekend confined to the East Coast.

Trump has spoken to both French President Emmanuel Macron and UK Prime Minister Theresa May. In two conversations in the space of two days, Macron and Trump continued "their coordination on responding to Syria's atrocious use of chemical weapons on April 7," the White House said.

One US official said the White House was most likely to coordinate with the French on a potential response since there are apparently concerns that May wouldn't receive approval from the UK parliament for strikes, CNN reported.

The United Nations Security Council failed to pass a resolution that would have established an independent investigation into the suspected use of chemical weapons in Syria, because of a veto by Russia. It was Russia's sixth veto related to chemical weapons in Syria.

'Tweeted himself into a corner'

Syrian activists on Saturday reported that helicopters had dropped barrel bombs carrying deadly gas onto the rebel-held Damascus suburb of Douma.

The images of injured and dying civilians prompted Trump to tweet about the "mindless chemical attack," and blame Russia and Iran for their support of President Bashar al-Assad. "Big price to pay," he said in an April 8 tweet

According to Anthony Blinken, former US deputy secretary of state and national security adviser to President Obama, Trump may have "tweeted himself into a corner, to some extent."

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"This attack has now jumbled the entire deck, and they now have to run as fast as they can to put some kind of plan in place to make good on the president's own rhetoric," Blinken said.

"He's the one who said in his own tweets that this is unacceptable he's going to have to take action and having taken action a year ago, he really can't do anything less so I think there's a mad race going on to figure out what that is."

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Russia and Iran vow retaliation

Unlike a year ago, when a chemical attack in Syria prompted Trump to order air strikes on a Syrian air base within days, the US military has held off on any immediate action.

One consideration is the potential response: On Wednesday, Russia repeated its warning that it would shoot down any US missiles.

The usual back-and-forth between the old Cold War foes has ensued, with Trump on Wednesday tweeting : "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!'"

Russian Foreign Ministry spokeswoman Maria Zakharova ridiculed Trump, saying on Facebook, "Smart missiles should fly toward terrorists."

Iran, meanwhile, has vowed to retaliate for a strike on a Syrian airbase that it blames on Israel. On Monday, Russia's Defense Ministry claimed that two Israeli F-15 warplanes launched eight guided missiles from Lebanese territory, targeting the T-4 base in central Syria. The Defense Ministry said Syrian defense units destroyed five of the incoming missiles.

A senior adviser to Ayatollah Ali Khamenei said Israel's "crimes will not remain unanswered."

The adviser, Ali Akbar Velayati, arrived in Damascus on Tuesday for an international conference. But Iran has long supported Assad, and Iran's state-run Islamic Republic News Agency reported that "some Iranian military advisers" had been killed in the missile strikes.

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Chemical weapons investigation

While military options are under consideration, a chemical weapons investigative team is on its way to Syria.

Inspectors from the Organization for the Prohibition of Chemical Weapons (OPCW) said they would send a fact-finding mission to Douma to investigate the suspected chemical attack.

Along with asking the Syrian government to "make the necessary arrangements for such a deployment," the team received a request from the Syrians to come and see for themselves.

The Syrian Foreign Ministry said it "looks forward to the mission's full transparency and reliance on concrete and credible evidence."

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But even as the agency prepared to deploy, it will not be able to attribute accountability to its findings, says the UN.

"The missing piece here is the accountability ... we have the fact-finding, but we need the other piece which was supplied in the past through the joint investigative mechanism which no longer exists," said Stéphane Dujarric, spokesperson for the UN Secretary General on Tuesday.

"We have half a tool. We have the tool through the OPCW that allows us to establish the facts. The facts are a very important part of it, but the other part is that of accountability."

Selecting appropriate targets

The European Aviation Safety Agency issued a safety warning on Tuesday, alerting airlines of the possibility of strikes into Syria "in the next 72 hours."

"Due to the possible launch of air strikes into Syria with air-to-ground and-or cruise missiles within the next 72 hours, and the possibility of intermittent disruption of radio navigation equipment, due consideration needs to be taken when planning fight operations in the Eastern Mediterranean-Nicosia area," the statement said. CNN was unable to clarify if the warning was based on news reports or specific military information.

When and if the strikes do come, they will need to be immeasurably more damaging to Syria than last year's to really be punitive, said Rodger Shanahan, a research fellow at the Lowy Institute in Australia.

"It has to be more significant than the one a year ago because you have to raise the cost to the Syrian government of undertaking these actions," he told CNN.

"The messaging has to be 'I've extracted a stronger cost on you than last time, if it happens again, I'll extract a larger cost,'" he said.

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But there's a limit to what the US can do.

Russia will respond immediately if its military in Syria gets hit by a possible US airstrike, first deputy chairman of the Russian upper house's Defense Committee, Yevgeny Serebrennikov said Wednesday according to Russia's state-run news agency Sputnik.

The US will likely notify Russians on the ground to avoid having their personnel near any targeted sites, and the strikes will likely not be too damaging that they change the balance of power on the ground, and give extremist groups on the ground the advantage.

So far though, the Syrians are winning.

On Tuesday the Russian military said 2,000 rebel fighters and their families left Douma, and that around 4,000 others were preparing to leave.

The area, which Syrian government forces have been slowly re-taking, had been the scene of countless and ineffective ceasefire proposals before the chemical attack.

The Russian military said on Tuesday that now, the situation in Douma was stabilizing, and that there had not been fighting there for several days.