The White House warned Syria's Bashar al-Assad on Monday that he and his military would "pay a heavy price" if it conducted a chemical weapons attack and said the United States had reason to believe such preparations were underway.

The White House said in a statement released late on Monday the preparations by Syria were similar to those undertaken before an April 4 chemical attack that killed dozens of civilians and prompted U.S. President Donald Trump to order a cruise missile strike on a Syrian air base.

"The United States has 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," White House spokesman Sean Spicer said.

"If ... Mr. Assad conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price," he said.

The Pentagon said Tuesday that the U.S. has seen chemical weapons activity at a Syrian air base that was used for an April sarin gas attack.

Capt. Jeff Davis, a Pentagon spokesman, said the activity at the Shayrat Air Base indicates "active preparations for chemical weapons use."

"This involved specific aircraft in a specific hangar, both of which we know to be associated with chemical weapons use," Davis said.

He added that the worrisome activity dated back to "the past day or two." He did not say how the United States collected its intelligence.

Trump, who took to Twitter not long after the statement went out, focused his attention on a Fox News report related to former President Barack Obama and the 2016 election rather than developments in Syria.

Trump ordered the strike on the Shayrat airfield in Syria in April in reaction to what Washington said was a poison gas attack by Assad's government that killed 87 people in rebel-held territory. Syria denied it carried out the attack.

Assad said in an interview with the AFP news agency earlier this year that the alleged April attack was "100 percent fabrication" used to justify a U.S. air strike.

The strike was the toughest direct U.S. action yet in Syria's six-year-old civil war, raising the risk of confrontation with Russia and Iran, Assad's two main military backers.

U.S. and allied intelligence officers had for some time identified several sites where they suspected the Assad government may have been hiding newly made chemical weapons from inspectors, said one U.S. official familiar with the intelligence.

The assessment was based in part on the locations, security surrounding the suspect sites and other information which the official, who spoke on condition of anonymity, declined to describe.

The White House warning, the official said, was based on new reports of what was described as abnormal activity that might be associated with preparations for a chemical attack.

Although the intelligence was not considered conclusive, the administration quickly decided to issue the public warning to the Assad regime about the consequences of another chemical attack on civilians in an attempt to deter such a strike, said the official, who declined to discuss the issue further.

At the time of the April strike, U.S. officials called the intervention a "one-off" intended to deter future chemical weapons attacks and not an expansion of the U.S. role in the Syrian war.

The United States has taken a series of actions over the past three months demonstrating its willingness to carry out strikes, mostly in self-defense, against Syrian government forces and their backers, including Iran.

The United States ambassador to the United Nations Nikki Haley said on Twitter: "Any further attacks done to the people of Syria will be blamed on Assad, but also on Russia and Iran who support him killing his own people."

Washington has repeatedly struck Iranian-backed militia and even shot down a drone threatening U.S.-led coalition forces since the April military strike. The U.S. military also shot down a Syrian jet earlier this month.

Trump has also ordered stepped-up military operations against the Daesh terror group and delegated more authority to his generals.