White House threatens 'heavy price' for Syria chemical strikes

The White House has warned Syrian President Bashar al-Assad that he and his military will "pay a heavy price" if a chemical weapons attack is carried out.

The US says it has reason to believe such preparations are underway.

The White House said in a statement 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 US 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.

White House officials did not respond to requests for comment on potential US plans or the intelligence that prompted the statement about Syria's preparations for an attack.

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 per cent fabrication" used to justify a US air strike.

The strike was the toughest direct US 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.

US 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 US 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.