Secretary of State Rex Tillerson said Thursday that “steps are underway” to build an international coalition to remove Syrian President Bashar al-Assad — as Pentagon officials prepared to brief President Trump on options for a military strike against his regime.

“Assad’s role in the future is uncertain, clearly, and with the acts that he has taken, it would seem that there would be no role for him to govern the Syrian people,” Tillerson said.

Both Tillerson and Trump singled out Assad as the one responsible for the poisonous gas attack this week that killed at least 86 people, including dozens of women and children.

Autopsies of several victims show they were exposed to sarin gas, a banned nerve agent that’s highly toxic, the Turkish Health Ministry said.

“There is no doubt in our minds, and the information we have supports that Syria, the Syria regime under the leadership of President Bashar al-Assad are responsible for this attack,” Tillerson said.

He added that the process to remove Assad would require “an international community effort.”

“Both to first defeat ISIS within Syria, to stabilize the Syrian country, to avoid further civil war, and then to work collectively with our partners around the world through a political process that would lead to Assad leaving,” Tillerson explained.

He warned Russian officials, with whom he’ll be meeting next week, to reconsider their allegiance to the Syrian strongman.

“It is very important that the Russian government consider carefully their continued support for the Assad regime,” he said.

The spokesman for Russian President Vladimir Putin, Dmitry Peskov, on Thursday said the Kremlin’s support for Assad was not unconditional.

“Unconditional support is not possible in this current world,” he said.

But he added that “it is not correct to say that Moscow can convince Mr. Assad to do whatever is wanted in Moscow. This is totally wrong.”

Trump, speaking aboard an Air Force One flight to Florida, where he met with Chinese President Xi Jinping, blasted the chemical attack in Syria as being “a disgrace to humanity.”

“What Assad did is terrible. 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,” he said.

“He’s [Assad] there, and I guess he’s running things, so I guess something should happen.”

Defense Department officials were drawing up plans for a military strike in retaliation for the chemical assault and Defense Secretary James Mattis was expected to brief Trump in Florida on his options, CNN reported.

The proposals were likely to include grounding Syrian aircraft and striking the country’s air defenses, according to NBC News.

Mideast experts told The Post that the US could undertake a limited military response with stealth bombers or cruise missiles.

“If military action is undertaken the US could target the units that were responsible for delivering the chemical attack, the airbase where the planes came from, the command and control center when the orders were given,” said Randa Slim, a senior fellow at the Middle East Institute in Washington.

“That is something that Russia could understand. They would not tolerate anything that would lead to large Russian casualties, or force the Assad regime to implode.”

The US already has a blueprint for a strike, as the Obama administration had drawn up an attack plan revolving around sea-launched cruise missiles after the 2013 chemical attack in Syria that left more than 1,400 civilians dead, The New York Times reported.

Instead of executing that strategy, Obama made a deal with Russia that supposedly eliminated Syria’s chemical weapon stockpile.

Sens. John McCain (R-Ariz.) and Lindsey Graham (R-SC) both called for the US to act, saying Assad “must pay a punitive cost for this horrific attack.”

“This is a test of the new administration, but also for our entire country,” the senators said in a statement. “Assad is trying to see what he can get away with. The rest of the region and the world is also watching to see how our country will respond, and what that means for them.”

They suggested the US lead an international coalition “to ground Assad’s air force.”

A major consideration for the US in its decision to take military action is how far Russia, which has been supporting the Syrian regime since 2015, would go to protect the government.

Early Thursday morning, Russia’s foreign ministry spokeswoman Maria Zakharova challenged Trump to lay out his retaliation strategy.

“Russia’s approach to Assad is clear. He is the legal president of an independent state. What is the US approach?” Zakharova asked CNN in a text message.

Moscow claimed a rebel arms depot struck by Syrian warplanes was the source of the toxic gas.

Syria blamed the attack on terrorists, insisting it had never used chemical weapons.

At the UN, the United States, which currently holds the presidency of the Security Council, drafted a resolution along with Britain and France that condemns the use of chemical weapons, particularly in the attack on the Syrian town of Khan Sheikhoun, “in the strongest terms.”

Russia objected to key provisions in the resolution and negotiations have been underway to try to bridge the differences.

Britain’s deputy ambassador Peter Wilson said “what we want is a unanimous resolution … and we want to see this done soon.”

Additional reporting by Mark Moore, Bob Fredericks and Post Wires