Syrian President Bashar al Assad appears to be preparing a fresh chemical weapons attack, the US has claimed.
In a statement, the White House said it had detected "potential preparations" similar to those made before a chemical attack on a rebel-held town in April, which killed at least 80 people.
It warned the use of chemical weapons "would likely result in mass murder of civilians, including innocent children", adding that Assad and his military would "pay a heavy price".
The White House did not provide evidence to back up the claim.
Nikki Haley, US ambassador to the United Nations, tweeted that any further chemical attacks "will be blamed on Assad, but also on Russia & Iran who support him killing his own people".
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.
— Nikki Haley (@nikkihaley) June 27, 2017
Reporter Cordelia Lynch said: "This is an ominous statement by the White House but it provides little detail.
"Statements like this are usually co-ordinated across national security agencies and yet, several military officials have suggested they were caught off guard by the White House release.
"It is unclear though how closely held the intelligence about a potential weapons attack was.
"It is also intriguing that as Sean Spicer, the Press Secretary raised the prospect of a potential chemical weapons attack and a US response, 41 minutes later, the President was tweeting about Obama."
The Syrian regime was widely blamed for the sarin gas attack in Khan Sheikhoun on April 4th, but it denied responsibility.
In retaliation, Donald Trump ordered missile strikes on a Syrian airbase and called on other nations to join in to help "end the slaughter and bloodshed".
Washington also sanctioned 271 employees of a Syrian government agency it blamed for developing chemical weapons.
At the time, US officials said the intervention was a "one-off" intended to deter future chemical weapons attacks and not an expansion of America's role in the Syrian war.
Last week, a US military jet shot down a Syrian warplane that dropped bombs near US-backed fighters in Tabqa, northern Syria.
The Assad regime later claimed the plane was on a combat mission against Islamic State fighters.
