President Donald Trump issued a stern warning to Syrian President Bashar Assad, claiming "potential" evidence that Syria was preparing for another chemical weapons attack.

In an ominous statement issued late Monday, Press Secretary Sean Spicer said the U.S. had "identified potential preparations for another chemical weapons attack by the Assad regime that would likely result in the mass murder of civilians, including innocent children."

He said the activities were similar to preparations taken before an April 2017 attack that killed dozens of men, women and children, and warned that if "Mr. Assad conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price."

To no surprise, a Syrian minister quickly responded to the statement, insisting his country will not use such arms.

But Capt. Jeff Davis, a Pentagon spokesman, confirmed the White House statement Tuesday, saying the activity at the Shayrat air base indicates "active preparations for chemical weapons use."

A non-governmental source with close ties to the White House said the administration had received intelligence that the Syrians were mixing precursor chemicals for a possible sarin gas attack in either the east or south of the country, where their Russian-backed troops have faced recent setbacks.

The chemical weapons attack in April shook the entire world, with horrifically graphic images pouring out of Syria.

Following the attack, Trump lambasted former President Barack Obama for not being tough enough on Syria in the past, and vowed to take action upon any indication of attempted mass murder of the Syrian people.

Days later, President Donald Trump ordered a retaliatory cruise missile strike on a Syrian government-controlled air base.