Comments by US president come as government forces prepare to launch an offensive to retake rebel-held Idlib province.

US President Donald Trump has warned the Syrian government against launching an attack on rebel-held Idlib province amid concerns that the planned offensive will create a new humanitarian crisis.

“President Bashar al-Assad of Syria must not recklessly attack Idlib Province. The Russians and Iranians would be making a grave humanitarian mistake to take part in this potential human tragedy,” Trump said in a tweet on Monday.

“Hundreds of thousands of people could be killed. Don’t let that happen.”

The warning came as Iran’s Foreign Minister Mohammad Javad Zarif met with the Syrian president in a surprise visit to Damascus ahead of an expected government offensive on Idlib.

The northern province is the last major rebel stronghold fighting al-Assad, who is backed by both Russian and Iranian forces.

Moscow and Tehran have insisted on fighting what they say are terrorist groups, with Zarif saying that areas under rebel control must be “cleaned out” and brought back under government authority.

Trump has sought better relations with Russia since taking office in 2017, but the United States has been unable to rein in Moscow’s military and diplomatic support for al-Assad.

US Secretary of State Mike Pompeo on Friday said Washington views any government assault on Idlib as an escalation of Syria’s war, and the State Department warned that Washington would respond to any chemical attack by Damascus.

Nikki Haley, the US ambassador to the United Nations, wrote on Twitter late on Monday: “All eyes on the actions of Assad, Russia, and Iran in Idlib. #NoChemicalWeapons”.

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Zarif’s visit to Damascus comes as Tehran prepares to host a summit on September 7 between the leaders of Iran, Russia and Turkey to discuss the situation in Idlib, Fars news agency reported.

Turkey, which has long supported anti-Assad rebels, has cooperated with Russia and Iran on talks over Syria in recent years and has troops in the Idlib region on an observation mission.

Last week, Iran’s defence minister travelled to Damascus and signed an agreement for defence cooperation between the two countries.