The White House has announced that Syrian President Bashar al-Assad is preparing to launch a chemical weapons attack on his own citizens, warning that the regime would “pay a heavy price” if it goes ahead.

In a statement released late on Monday, the White House said 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.

'Dozens killed' in suspected chemical attack in Syrian province

“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.

Mr 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.

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

Mr 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 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, Mr 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.

Although the intelligence was not considered conclusive, the administration quickly decided to issue the public warning to Mr Assad’s 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, US officials called the intervention a “one-off” intended to deter future chemical weapons attacks and not an expansion of the US role in the Syrian war.

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The United States has taken a series of actions over the past three months demonstrating its willingness to carry out strikes, mostly in self-defence, 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 US-led coalition forces since the April military strike.

The US military also shot down a Syrian jet earlier this month.

Mr Trump has also stepped up military operations against the Islamic State militant group and delegated more authority to his generals.