WASHINGTON - The White House issued a stern warning to Syrian President Bashar Assad on Monday night as it claimed "potential" evidence that Syria was preparing for another chemical weapons attack.

In an ominous statement issued with no supporting evidence or further explanation, Press Secretary Sean Spicer said the U.S. "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."

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."

Assad had denied responsibility for an April 4 attack on the town of Khan Sheikhoun in the rebel-held Idlib province that killed dozens of people, including children. Victims show signs of suffocation, convulsions, foaming at the mouth and pupil constriction.

Days later, U.S. President Donald Trump launched a cruise missile strike on a Syrian government-controlled air base where U.S. officials said the Syrian military had launched the chemical attack.

It was the first direct American assault on the Syrian government and Trump's most dramatic military order since becoming president months before.

Trump said at the time that the Khan Sheikhoun attack crossed "many, many lines," and put the blame squarely on Assad's forces.

Syria, meanwhile, maintained it hadn't used chemical weapons and blamed opposition fighters for stockpiling the chemicals. Russia's Defence Ministry said the toxic agents were released when a Syrian airstrike hit a rebel chemical weapons arsenal and munitions factory.

The U.S. attack on a Syrian air base came after years of heated debate and deliberation in Washington over intervention in the bloody civil war.

Chemical weapons have killed hundreds of people since the start of the conflict, with the UN blaming three attacks on the Syrian government and a fourth on the Islamic State group.

Nikki Haley, the United States Ambassador to the United Nations, added Monday on Twitter: "Any further attacks done to the people of Syria will be blamed on Asaad, but also on Russia & Iran who support him killing his own people."

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Associated Press writers Josh Lederman, Vivian Salama and Matthew Lee contributed to this report.