President Trump is signaling a more aggressive approach toward Syria, saying on Wednesday reports of a deadly gas attack by forces loyal to Bashar Assad had “crossed a lot of lines” and moved him to rethink his strategy.

At a joint press conference on Wednesday at the White House with Jordan’s King Abdullah II, Trump indicated that the Syrian leader had gone too far with an attack this week that killed dozens of his own people and injured scores more, including women and children.

“It crossed a lot of lines for me,” Trump said. “When you kill innocent children, innocent babies, little babies with a chemical gas that is so lethal that people were shocked to hear what gas it was, that crosses many lines beyond the red line. Many, many lines.”

“I will tell you, what happened yesterday is unacceptable to me,” Trump added.

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The strike on the rebel-held province of Idlib is being described as the worst chemical weapons attack in years, with human rights groups estimating that 72 people have died, including 20 children. The attack is believed to have been carried out by forces loyal to Assad, although the Syrian military has denied using chemical weapons.

Gruesome video footage has emerged showing Syrian children struggling to breathe from exposure to what is believed to be the nerve agent Sarin. Those images have provoked international outrage and an emergency meeting by the United Nations Security Council.

Trump has in the past warned that the U.S. should not get involved in the Syrian civil war, even while he has hammered former President Obama for backing away from his “red line” over Assad’s use of chemical weapons.

For example, Trump tweeted in 2013 that it would be “foolish” to intervene in the civil war and would bring nothing to the U.S.

“President Obama, do not attack Syria,” Trump tweeted. “There is no upside and tremendous downside. Save your 'powder' for another (and more important) day!”

But the president on Wednesday struck an emotional tone at the Rose Garden press conference, saying the reports of women and children who had died had a "big impact" on him and caused him to rethink his strategy toward Assad.

“I do change. I am flexible. I am proud of that flexibility,” Trump said. “I will tell you that attack on children yesterday had a big impact on me. Big impact. It was a horrible, horrible thing. I've been watching it and seeing it, and it does not get any worse than that. I have that flexibility. And it is very, very possible, and I will tell you it is already happened, that my attitude toward Syria and Assad has changed very much.”

Trump declined to outline what his new approach to Syria might entail.

The developments pose a vexing challenge for the administration, which would prefer to keep its focus on fighting the Islamic State in Iraq and Syria, rather than meddling in a foreign civil war.

Secretary of State Rex Tillerson has called on Assad’s allies in Russia and Iran to intervene and discourage the Syrian leader from using chemical weapons on his own people.

Tillerson has given no indication that the U.S. might play a role in Assad's removal, saying days ago that his “longer term status…will be decided by the Syrian people."

“We call upon Russia and Iran, yet again, to exercise their influence over the Syrian regime and to guarantee that this sort of horrific attack never happens again,” Tillerson said in a statement. “Russia and Iran also bear great moral responsibility for these deaths.”

White House press secretary Sean Spicer said Tuesday that Assad’s removal from power is “in the best interest of the Syrian people,” but Tillerson’s remarks “speak to the political realities of the situation.”

Trump on Wednesday also continued to cast blame on his predecessor for the instability, accusing former President Obama of creating the conditions that led to the gas attack by backing away from his “red line” threat against Assad for the use of chemical weapons.

“I think the Obama admission had a great opportunity to solve this crisis a long time ago when he said the red line in the sand,” Trump said. “When he did not cross that line after making the threat, I think that set us back a long ways, not only in Syria but in many other parts of the world because it was a blank threat. I think it was something that was not one of our better days as a country.”

Trump said , however, that the responsibility for Syria is now his own.

“I now have responsibility, and I will have that responsibility and carry it very proudly,” he said. “I will tell you that. It is now my responsibility.”

--This report was updated at 3:00 p.m.