The White House has issued a warning to Bashar Assad as it claimed "potential" evidence showed 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 US "has identified potential preparations" for another chemical attack by Bashar Assad's government that "would likely result in the mass murder of civilians, including innocent children".
Mr Spicer said the activities were similar to preparations taken before a chemical attack in April, 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".
The White House offered no details on what prompted the warning and spokeswoman Sarah Sanders said she had no additional information.
Several State Department officials typically involved in co-ordinating such announcements said they were caught completely off guard by the warning, which did not appear to be discussed in advance with other national security agencies.
Typically, the State Department, the Pentagon and US intelligence agencies would be consulted before the White House issued a declaration sure to reverberate across foreign capitals.
A non-governmental source with close ties to the White House said the administration had received intelligence that the Syrians were mixing precursor chemicals for a possible sarin gas attack in either the east or south of the country, where government troops and their proxies have faced recent setbacks.
President Assad denied responsibility for the April 4 attack on the town of Khan Sheikhoun in the rebel-held Idlib province.
Victims showed signs of suffocation, convulsions, foaming at the mouth and pupil constriction.
Days later, US president Donald Trump launched a cruise missile strike on a Syrian government-controlled air base where American officials said the chemical attack was launched.
It was the first direct American assault on the Syrian government and Mr Trump's most dramatic military order since becoming president months before.
Mr Trump said at the time that the Khan Sheikhoun attack crossed "many, many lines" and put the blame squarely on Assad's forces.
Syria maintained it had not used chemical weapons and blamed opposition fighters for stockpiling the chemicals.
Russia's defence ministry said the toxic agents were released when a Syrian air strike hit a rebel chemical weapons arsenal and munitions factory.
The US attack on the 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 since the start of the conflict, with the United Nations blaming three attacks on the Syrian government and a fourth on the Islamic State terror group.
Earlier on Monday, Mr Trump had dinner with secretary of state Rex Tillerson, defence secretary Jim Mattis, national security adviser HR McMaster and other top officials as he hosted a visit by Indian prime minister Narendra Modi at the White House.
Mr Tillerson and Russian foreign minister Sergey Lavrov talked earlier about the need to secure a ceasefire in Syria, fight extremist groups and prevent the use of chemical weapons, Russia's Foreign Ministry said.
On Monday, Nikki Haley, the United States ambassador to the United Nations, said on Twitter: "Any further attacks done to the people of Syria will be blamed on Assad, but also on Russia & Iran who support him killing his own people."