PALM BEACH, Fla. — The Trump administration is developing plans to respond to Syria's apparent use of chemical weapons, including possible military action and attempts to remove Syrian President Bashar al-Assad from power, Trump and White House aides said Thursday.

“I don’t want to say what I’m going to be doing with respect to Syria,” Trump told reporters en route to his Mar-a-Lago estate in Palm Beach, Fla., where he will host Chinese counterpart Xi Jinping for a summit over the next two days.

The increased activity about Syria represents an about-face for Trump, who had previously urged the United States to support Assad against the rebel groups fighting him, many which are aligned with al-Qaeda. Assad is also backed by Russia and Iran. Tuesday's attack on a town in the northern Syrian province of Idlib shifted Trump's outlook on Syria, spurring him to consider new options against the nation wracked by civil war.

Asked about whether Assad should step down, Trump said  “I think what happened in Syria is a disgrace to humanity, and he’s there, and I guess he’s running things, so I guess something should happen.

"What Assad did is terrible," Trump said. "What happened in Syria is truly one of the egregious crimes and it shouldn’t have happened. And it shouldn’t be allow to happen.”

Trump "is being presented with a lot of options," White House spokesman Sean Spicer said, adding that Trump has not signed off on any specific plans.

While Trump held off on saying whether the United States would lead a global effort to remove Assad from power, Secretary of State Rex Tillerson said "those steps are underway."

Horrified by photos

Speaking to reporters in Palm Beach, where Trump is meeting with the president of China, Tillerson said the pictures from Syria "just horrified all of us.” He said that, given the evidence, there is "no doubt in our minds" that "the Syrian regime, under the leadership of Bashar al-Assad, is responsible for this attack.”

He added "there is no room for him to govern the Syrian people," though any process for forcing out Assad "would require international effort.”

To that end, Tillerson said Russia “should reconsider carefully” its role in supporting the Assad regime.

“We are considering an appropriate response,” he said. “It’s a serious matter that requires a serious response.”

Also meeting with Trump in Florida are Defense Secretary Jim Mattis, National Security Adviser H.R. McMaster, who are developing a response with deputies K.T. McFarland and Dina Powell.

On Wednesday, Trump condemned the attack that claimed the lives of innocent women and children, and "these heinous actions by the Assad regime cannot be tolerated." The attack "crossed a lot of lines for me," Trump said after meeting with a key Middle East ally, King Abdullah of Jordan.

Officials said Trump has spoken with world leaders about the establishment of "safe zones" for Syrian citizens caught in the middle of the nation's civil war.

Sen. John McCain, R-Ariz., said that Trump told him he would be consulting with Mattis and McMaster, a Mattis protege.

McCain and Sen. Lindsey Graham, R-S.C., have called for action that would include grounding Assad's air force, thereby denying him "a strategic advantage in his brutal slaughter of innocent civilians, both through the use of chemical weapons as well as barrel bombs, which kill far more men, women and children on a daily basis."

Saying that the world is "watching to see how our country will respond," McCain and Graham said in a joint statement that "if the President is willing to take the necessary action, he deserves broad bipartisan support, and we will help build it.”

Possibilities range from seeking increased economic sanctions to military action, aides said, though Trump has opposed that latter alternative in years past.

A change of heart

In January, Trump and Russian President Vladimir Putin spoke on the telephone on Syria and other issues. In that call, they agreed to seek ways to join forces to fight the Islamic State terrorist group in Syria along with Assad.

The White House put out a short statement later on the one-hour phone call, saying it "ranged in topics from mutual cooperation in defeating ISIS to efforts in working together to achieve more peace throughout the world including Syria."

It added that "the positive call was a significant start to improving the relationship between the United States and Russia that is in need of repair."

At the time, McCain warned that Russia could not be trusted, because it was propping up a "murderous regime" in Syria.