President Trump vowed Monday that the United States would take swift action in retaliation for a suspected chemical weapons attack on civilians in Syria, amid hurried diplomacy that signaled allied military strikes may be imminent.

The Trump administration, backed by France and Britain, began making a circumstantial case that Syria and its Russian and Iranian partners bear direct responsibility for the weekend deaths of at least 49 people in the opposition-held town of Douma, outside the Syrian capital, Damascus.

“It was an atrocious attack. It was horrible,” Trump said at the start of a Cabinet meeting that was one of several White House gatherings Monday where possible military action was discussed.

Options include the sort of largely symbolic airstrike Trump ordered a year ago in response to a similar chemical attack blamed on Syrian President Bashar al-Assad, or a wider and riskier assault.

“We are studying that situation extremely closely. We are meeting with our military and everybody else, and we’ll be making some major decisions over the next 24 to 48 hours,” Trump said.

“We are very concerned when a thing like that can happen,” the president said in a somber tone. “This is about humanity. We’re talking about humanity. And it can’t be allowed to happen.”

Trump said “nothing is off the table” in responding to the attack, suggesting that a new round of airstrikes could come quickly. Any military response that would involve significant U.S. ground forces remains unlikely.

The White House had earlier suggested that the United States and France could act together, saying that during a phone call late Sunday, Trump and French President Emmanuel Macron had agreed to “coordinate a strong, joint response.”

“If it’s Russia, if it’s Syria, if it’s Iran, if it’s all of them together, we’ll figure it out and we’ll know the answers quite soon,” Trump said.

Later Monday, Trump met with military leaders over dinner and told reporters that “we’re going to make a decision tonight or very shortly thereafter.”

“It will be met and it will be met forcefully,” he said of the attack. “When, I will not say, because I don’t like talking about timing.”

Trump said Sunday on Twitter that there would be a “big price to pay” for the attack — a change in tone from last week, when he said he intended to pull U.S. forces out of Syria as fast as possible.

After Trump’s remarks about a U.S. withdrawal, Sen. John McCain (R-Ariz.), among other critics, accused the president of emboldening Assad and inviting the kind of attack launched in Douma.

At the United Nations, U.S. Ambassador Nikki Haley and her French and British counterparts challenged the U.N. Security Council to resume an independent investigation of Syrian chemical weapons use.

“History will record this as the moment when the Security Council either discharged its duty or demonstrated its utter and complete failure to protect the people of Syria,” she said. “Either way, the United States will respond.”

Haley accused Russia of helping Assad avoid international accountability. She called Assad a “monster” incapable of remorse and said his Russian enablers are incapable of shame.

Vassily Nebenzia, Russia’s U.N. representative, told the Security Council that images of the dead in Douma were staged and “fake news.”

Nebenzia accused Washington of employing slander, hawkish rhetoric and blackmail beyond what was practiced during the Cold War.

“Do you understand the dangerous threshold to which you are bringing the world?” he asked.

Both the Russian and Syrian envoys promised to cooperate with investigators from the Organization for the Prohibition of Chemical Weapons if they travel to Syria to take samples and talk to victims.

Bashar Jaafari, the Syrian ambassador, predicted they would find no evidence that chemical weapons were used.

“Syria does not possess chemical weapons of any kind,” he said. “We condemn their use, at any time and under any circumstances.”

Any military action by the United States, France, Britain or others would be predicated on claims that Syria deliberately targeted civilians, which Syria denies.

“We’ll be looking at that barbaric act and studying what’s going on. We’re trying to get people in there,” Trump said, referring to the site of the attack. “As you know, it’s been surrounded. So it’s very hard to get people in because not only has it been hit, it’s been surrounded. And if they’re innocent, why aren’t they allowing people to go in and prove?”

Asked by a reporter if he had any doubt who was behind the attack, the president said, “To me there’s not much a doubt, but the generals will figure it out.”

White House press secretary Sarah Huckabee Sanders would not provide details to back up U.S. assertions that Syria is to blame, but she said U.S. officials are “very confident” in making the claim.

The images of families and children lying dead together, eyes glassy and foam on their lips, appear to have had a strong effect on Trump.

“You don’t see things like that,” he said Monday. “As bad as the news is around the world, you just don’t see those images.”

That echoed what Trump had said last April, when he ordered limited airstrikes on a military site in response to an alleged sarin attack in northwest Syria that killed civilians, including children.

The strike, which involved 59 cruise missiles launched from U.S. destroyers in the Mediterranean, was the first direct U.S. assault against the Assad government since the war began. It targeted the Sharyat air base in Homs province, which officials said had been used by a Syrian-piloted, Russian-made SU-22 plane that dropped the lethal agent on a civilian area, killing dozens of people.

But critics said the U.S. action, which targeted air defenses, aircraft, fuel and hangars at an isolated and sparsely populated military facility, was too limited to have a lasting deterrent effect on the Syrian leader, who has regained the upper hand against opposition groups with Russian and Iranian military support.

The strike’s significance faded in the subsequent months. The Assad government, according to U.S. assessments, has continued making and using chemical agents, in part to compensate for its dwindling conventional military might.

Defense Secretary Jim Mattis warned after the 2017 strike that Syria would pay a “very, very, very stiff price” for further chemical attacks. But military leaders have opposed actions they fear would drag them into Syria’s chaotic civil war, which involves Syrian, Russian, Iranian and Turkish military elements as well as various militias.

Speaking Monday before meeting with the emir of Qatar, Mattis said he would not rule out any kind of response to the latest incident in Syria.

“The first thing we have to look at is why are chemical weapons still being used at all when Russia was the framework guarantor of removing all the chemical weapons,” he said. “Working with our allies and our partners from NATO to Qatar and elsewhere, we are going to address this issue.”

There have been 10 chemical attacks reported in Syria this year, including the Douma incident, according to the Syrian American Medical Society.

John Wagner and Missy Ryan contributed to this report.