WASHINGTON – The White House announced Monday that the United States has detected possible preparations by Syrian President Bashar Assad's regime to carry out another chemical attack against its own civilians.

A statement released by Press Secretary Sean Spicer warned Assad of the "heavy price" his regime will pay for such action.

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"As we have previously stated, the United States is in Syria to eliminate the Islamic State of Iraq and Syria. If, however, Mr. Assad conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price," the statement said.

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

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 at least 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. airstrike.

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.

'Abnormal activity'

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 that 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.

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 Islamic State militant group and delegated more authority to his generals.