A brutal chemical-weapons attack on Syrian civilians is posing a major test for U.S. President Donald Trump, who reacted with outrage to the killings but declined to say what actions the U.S. might take in response.

Mr. Trump blamed Syrian President Bashar al-Assad for Tuesday's atrocity, which killed more than 70 people, including children. "These heinous actions by the Assad regime cannot be tolerated," he said, calling the killings "an affront to humanity."

At an emergency session of the United Nations Security Council on Wednesday, U.S. Ambassador Nikki Haley held up photos of children killed in the attack. Ms. Haley delivered a withering critique of Russia, Mr. al-Assad's staunch ally. "How many more children have to die before Russia cares?" she asked.

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Explainer: A look at Syria's history with chemical weapons and how the international community can help

If the UN consistently fails to respond in a collective manner to Syria's use of chemical weapons, then "there are times in the life of states when we are compelled to take our own action," Ms. Haley said.

In Canada, Prime Minister Justin Trudeau described the attack as a war crime. "We are shocked and appalled at the reports of chemical-weapon attacks against civilians in Syria," he told the House of Commons on Wednesday. "It is critical that we hold those responsible to account," he added, noting that Canada was "supporting evidence-gathering to achieve that end."

Mr. Trump has lambasted his predecessor's approach to the long-running conflict in Syria as weak, irresolute and detrimental to American interests. Former president Barack Obama eschewed military action in the context of what he considered an intractable, multisided civil war that has drawn in players from the region and beyond.

But now, Mr. Trump finds himself in a situation that is an echo of what Mr. Obama faced in 2013, when Mr. Assad deployed chemical weapons in a devastating attack that killed more than 1,400 people outside Damascus.

The dilemma has not changed, said Robert Ford, a former U.S. ambassador to Syria under Mr. Obama. "The Trump administration needs to decide whether or not they want to establish a deterrent" through the use of limited air strikes, said Mr. Ford, since sanctions and international condemnation have failed to dissuade the Assad regime from using chemical weapons.

The risk is that such air strikes could lead to further escalation, Mr. Ford said. But the alternative is that "the Syrian government acts with complete impunity and an international norm is consistently violated."

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The Trump administration's tough language and promises of a strong and possibly unilateral response to Tuesday's attack suggest that it is weighing a departure from the Obama-era approach to Syria.

While Mr. Trump has criticized Mr. Obama's policies in Syria, he has also repeatedly said that the United States should avoid getting sucked into the conflict and instead focus on solving problems at home.

However, Mr. Trump said that Tuesday's attack "had a big impact on me … it doesn't get any worse than that." He added that his "attitude toward Syria and Assad has changed very much" and said he was willing to use "flexibility" in his approach to the conflict.

The attack "crossed a lot of lines for me," the President said during a news conference Wednesday at the White House with King Abdullah of Jordan. "When you kill innocent children, innocent babies, little babies, with a chemical gas … that crosses many, many lines."

He declined to say if or how the United States would use force to punish Mr. al-Assad. "One of the things you've noticed about me militarily, I don't like to say where I'm going and what I'm doing," Mr. Trump said. "I'm not saying I'm doing anything one way or the other, but I'm certainly not going to be telling you."

Mr. Trump once again criticized Mr. Obama for failing to respond with force after the chemical-weapons attack by the Assad regime in 2013. Mr. Obama had previously warned Mr. al-Assad that any use of such weapons would be crossing a "red line" for the United States.

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"It was a blank threat," Mr. Trump said Wednesday. "That set us back a long ways, not only in Syria but in many other parts of the world."

(Mr. Trump's own views on the subject have shifted: Back in 2013, he urged Mr. Obama not to respond with military force to the chemical-weapons attack, writing on Twitter, "Do not attack Syria, if you do, many bad things will happen." In the end, the Obama administration brokered a deal with Russia to dispose of Syrian chemical weapons.)

Shadi Hamid, an expert on the Middle East at the Brookings Institution in Washington, said that the President faces the choice of sticking with a purely rhetorical response or resorting to military strikes. Mr. Hamid said he favoured the latter, but only in the context of a broader strategic vision for how to end the Syrian conflict and use military force to pressure the Assad regime into serious negotiations.

"It can't just be, 'We're pissed off about this and we're going to launch some air strikes and then we're done and we're back to the status-quo ante,'" Mr. Hamid said.

With a report from Bill Curry in Ottawa