AFP- Iran warned the United States on Tuesday against a "dangerous" escalation in Syria after Washington accused President Bashar al-Assad of possibly preparing another chemical attack for which he would pay a "heavy price".

"Another dangerous US escalation in Syria on fake pretext will only serve ISIS (the Islamic State group), precisely when it's being wiped out by Iraqi and Syrian people," tweeted Foreign Minister Mohammad Javad Zarif.

Iran is the key political and military backer of Assad alongside Russia, which has also criticised Washington's "threats". White House spokesman Sean Spicer on Monday said in a statement that the US "has identified potential preparations for another chemical weapons attack by the Assad regime that would likely result in the mass murder of civilians."

"If... Mr Assad conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price," the statement said. The US launched a cruise missile strike on a Syrian air base in April following a chemical weapons attack allegedly carried out by Assad's regime.

Iran is helping in the fight against the Islamic State group in Syria and Iraq, but its support for Assad has put it fiercely at odds with Western countries.