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 earlier this year, 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 UN, tweeted that any further chemical attacks "will be blamed on Assad, but also on Russia & Iran who support him killing his own people".


Moscow dismissed the US warning.

President Vladimir Putin's spokesman, Dmitry Peskov, said: "Such threats to Syria's legitimate leaders are unacceptable."

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The Syrian regime was widely blamed for the sarin gas attack in Khan Sheikhoun on 4 April, 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".

British Defence Secretary Sir Michael Fallon has said Britain would support the US if Washington decided to take action again against Syria.

"The use of chemical weapons against innocent civilians is absolutely abhorrent, it's illegal under any rule of war," he told Sky News.

"We fully supported the last strike that took out the airplanes and the support they required to drop chemical weapons, and if the US is considering as similar strike, then we'll support that, too."

Asked if British aircraft or aircrew would be involved, Sir Michael said: "We've not been asked to be involved in these strikes, we weren't asked the last time. So that's hypothetical.

"But I want to leave you in no doubt, we think it's absolutely right, if we can, to prevent the use of chemical weapons, and we will fully support any US action."

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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.

Sky News US correspondent 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."

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.