WASHINGTON >> His expression grave and his words emphatic, President Donald Trump declared today the deadly chemical attack in Syria had crossed “many, many lines” and abruptly changed his views of Syrian President Bashar Assad. But he refused to say what the U.S. might do in response.

Trump issued no ultimatums in comments that were being scoured by world leaders for signs of how the new president would react to a global crisis. In a rare reversal of roles, Trump was more reserved than many of his top advisers — including his U.N. envoy who revived the hard-hitting rhetoric of Trump’s political campaign and strongly hinted some U.S. action was coming.

Trump himself was noncommittal.

“I’m not saying I’m doing anything one way or another, but I’m certainly not going to be telling you,” he told reporters.

He blamed the attack squarely on Assad’s forces, though the embattled Syrian leader and his Russian backers denied it. He suggested that the assault that killed 72 people had diminished his former reluctance to plunge the U.S. further into the complex and dangerous turmoil in the Middle East.

“When you kill innocent children, innocent babies — babies, little babies — with a chemical gas that is so lethal, people were shocked to hear what gas it was, that crosses many, many lines,” Trump said in the White House Rose Garden. U.S. officials said the gas was likely chlorine, with traces of a nerve agent like sarin.

While continuing to fault predecessor Barack Obama for much of the current situation in Syria, he acknowledged that dealing with the crisis is now his own responsibility and vowed to “carry it very proudly.”

Only days earlier multiple members of Trump’s administration had said Assad’s ouster was no longer a U.S. priority, drawing outrage from Assad critics in the U.S. and abroad. But Trump said Tuesday’s attack “had a big impact on me — big impact.”

“My attitude towards Syria and Assad has changed very much,” he said.

Since the attack Tuesday in rebel-held territory in northern Syria, Trump has been under increasing pressure to explain whether the attack would bring a U.S. response. After all, Trump’s first reaction was merely to blame Obama’s “weakness” in earlier years for enabling Assad.

Obama had put Assad on notice that using chemical weapons would cross a “red line” necessitating a U.S. response, but then failed to follow through, pulling back from planned airstrikes after Congress wouldn’t vote to approve them. Trump and other critics have cited that as a key moment the U.S. lost much global credibility.

“I now have responsibility,” Trump said. “That responsibility could be made a lot easier if it was handled years ago.”

Yet he was adamant that he would not telegraph any potential U.S. military retaliation, saying anew that that was a mistake the Obama administration had repeatedly made.

Standing alongside Jordan’s King Abdullah II at a news conference, Trump appeared to adopt the first part of Obama’s stance — that chemical weapons use is intolerable — while stopping short of saying what might come next.

That left some Assad opponents wanting more.

“It’s simply impossible to shame the Assad and (Russian President Vladimir) Putin regimes with words alone,” said Sen. Lindsey Graham, R-S.C.

The strongest indication that the U.S. might act came at the United Nations, where U.N. Ambassador Nikki Haley held up photos of the attack’s victims in an emotional plea to the Security Council to intervene.

“When the United Nations consistently fails in its duty to act collectively, there are times in the life of states that we are compelled to take our own action,” Haley declared.

Though Trump didn’t mention it, both Haley and Secretary of State Rex Tillerson have argued that Russia and Iran — Assad’s two staunchest allies — must use their influence to prevent him from mounting further attacks. Haley accused Moscow of blocking Security Council action and closing its eyes to “barbarity” by ignoring previous chemical assaults.

The most recent attack, in the town of Khan Sheikhoun, bore telltale signs of nerve agent exposure such as victims convulsing and foaming from the mouth. Videos showed volunteer medics using firehoses to wash chemicals from victims’ bodies and lifeless children being piled in heaps.

Early U.S. assessments show the attack most likely involved chlorine and traces of the nerve agent sarin, according to two U.S. officials, who weren’t authorized to speak publicly about intelligence assessments and demanded anonymity. Use of sarin would be especially troubling because it would suggest Syria may have cheated on its previous deal to give up chemical weapons.

After a 2013 attack, the U.S. and Russia brokered a deal in which Syria declared its chemical weapons arsenal and agreed to destroy it. Chlorine, which has legitimate uses as well, isn’t banned except when used in a weapon. But nerve agents like sarin are banned in all circumstances.

As Trump and other world leaders scrambled for a response, the U.S. was working to lock down details proving Assad’s culpability. Russia’s military, insisting Assad wasn’t responsible, has said the chemicals were dispersed when a Syrian military strike hit a facility where the rebels were manufacturing weapons for use in Iraq.

An American review of radar and other assessments showed Syrian aircraft flying in the area at the time of the attack, a U.S. official said. Russian and U.S. coalition aircraft were not there, the official said.

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Associated Press writers Vivian Salama, Ken Thomas, Lolita C. Baldor and Bradley Klapper in Washington, Vladimir Isachenkov in Moscow, and Edith M. Lederer at the United Nations contributed to this report.