The US and Russia have exchanged barbed words ahead of an operation in rebel-held Idlib, the last opposition stronghold in Syria.

Donald Trump had warned Russia, Syria and Iran that they would be making a “grave humanitarian mistake” if they go ahead with an assault on Idlib. Kremlin spokesman Dmitry Peskov meanwhile accused the White House of a one-sided approach that threatened to destabilise the region.

Speaking at his daily press conference, Mr Peskov said that Mr Trump's warning failed to take into account "the potential for further negative consequences in Syria." He claimed Idlib had become a "nest of terrorists" who had become a "threat" to Russian forces temporarily based in the region.

President Bashar al-Assad and his allies have been amassing a huge military presence near to rebel-held Idlib, where opposition fighters and Islamist groups are holding out among a population of around 3 million people.

Mr Trump tweeted on Monday night that Syria’s President Bashar al-Assad “must not recklessly attack Idlib Province”.

He added: “The Russians and Iranians would be making a grave humanitarian mistake to take part in this potential human tragedy.”

The UN said last week it was bracing for “the most horrific tragedy” in Idlib and dubbed it a “dumping ground” for fighters and civilians who have been pushed out of successive pockets of opposition-held territory.

Staffan de Mistura, the UN envoy for Syria, warned as many as 800,000 people could be displaced if the fighting does begin.

In his tweet, the US president went one step further, suggesting “hundreds of thousands of people could be killed”. That might seem unlikely, given an estimated death toll of between 400,000 and half a million for the entire seven-year duration of the Syrian civil war.

Nonetheless, Syria and its allies have been ramping up the aggressive rhetoric towards Idlib. Mr Trump’s tweet came as Iran's foreign minister Mohammad Javad Zarif said at the start of a visit to Damascus that "terrorists must be purged” from the province and the entire of Idlib returned to government control.

"Syria's territorial integrity should be safeguarded and all tribes and groups, as one society, should start the reconstruction process, and the refugees should return to their homes," Mr Zarif said.

He met with Mr Assad and his foreign minister Walid al-Moallem, who is just back from a visit to Moscow.

The visit comes days before the leaders of Iran, Turkey, and Russia are expected to meet in Iran to discuss the situation in Idlib.

During their meeting, Mr Assad and Mr Zarif discussed the agenda of the summit in Iran.

A statement from Mr Assad's office said Iran and Syria "had similar views on the different issues" to be discussed. It provided no further details.

Mr Zarif said it was necessary to consult "with our Syrian friends" ahead of the 7 September summit, according to the Fars news agency.

Iran has lent crucial military and economic support to Mr Assad throughout the civil war and the discussions are expected to focus on the decisive battle for Idlib.

Mr Assad has vowed to defeat the opposition in its last refuge in the north-western province if the rebels do not surrender to government rule.

Nearly half of the people currently living in Idlib have already been displaced more than once by the civil war.

Tens of thousands fled there after surrendering, choosing to relocate to an opposition-held area rather than risk reprisals or forced conscription at the hands of the government.

In their meeting on Monday, Mr Assad and Mr Zarif also discussed what they called "western pressure" on their two countries, in apparent reference to the US sanctions on Iran and calls for limiting Iran's role in Syria.

"War of Annihilation" – devastating toll on civilians in Raqqa, Syria Show all 20 1 /20 "War of Annihilation" – devastating toll on civilians in Raqqa, Syria "War of Annihilation" – devastating toll on civilians in Raqqa, Syria A child injured by an IED explosion in Raqqa Amnesty International "War of Annihilation" – devastating toll on civilians in Raqqa, Syria Children riding a bicycle among destroyed buildings in Raqqa Amnesty International "War of Annihilation" – devastating toll on civilians in Raqqa, Syria Entire neighbourhoods in Raqqa are damaged beyond repair Amnesty International "War of Annihilation" – devastating toll on civilians in Raqqa, Syria Satellite image showing the house where seven members of the Badran family were killed in a Coalition strike on 18 July 2017, before the strike CNES/Airbus DS "War of Annihilation" – devastating toll on civilians in Raqqa, Syria Satellite image showing the house where seven members of the Badran family were killed in a Coalition strike on 18 July 2017, after the strike CNES/Airbus DS "War of Annihilation" – devastating toll on civilians in Raqqa, Syria Members of the Badran family killed in three separate Coalition air strike on 18 July and 20 August 2017 in Raqqa Amnesty International "War of Annihilation" – devastating toll on civilians in Raqqa, Syria The city of Raqqa in northern Syria has become hardly recognizable to those who try to return after months battle between US-backed forces and the Islamic State terrorist group for control of it. The city has no running water or electricity and homes, business and public squares are mere rubble and debris Amnesty International "War of Annihilation" – devastating toll on civilians in Raqqa, Syria Satellite image showing the houses where 16 members of the Fayad family and neighbours were killed in Coalition strikes on 12 October 2017, before the strike CNES/Airbus DS "War of Annihilation" – devastating toll on civilians in Raqqa, Syria Satellite image showing the houses where 16 members of the Fayad family and neighbours were killed in Coalition strikes on 12 October 2017, after the strike CNES/Airbus DS "War of Annihilation" – devastating toll on civilians in Raqqa, Syria Fayad Mohammed and his daughters Wafa’, Fadda and Tamam; Ammar al-Faris; Reem al-Maddad, Yusra Abd-al-Aziz, and baby Razqiya Habib; Jasim Hamal and Salem Hamad. They were among the 16 civilians killed in Coalition air strikes on 12 October 2017 in Raqqa Amnesty International "War of Annihilation" – devastating toll on civilians in Raqqa, Syria Men wait by the side of the road for casual labour in Raqqa. Many end up clearing partially destroyed or damaged buildings, a very risky endeavour as many building were mined by IS and civilians are frequently killed and injured by mines Amnesty "War of Annihilation" – devastating toll on civilians in Raqqa, Syria Satellite images showing the Aswad family’s building before it was destroyed in a Coalition air strike which killed eight civilians, five of them children, on 28 June 2017 CNES/Airbus DS "War of Annihilation" – devastating toll on civilians in Raqqa, Syria Satellite image showing the Aswad family’s building after it was destroyed in a Coalition air strike which killed eight civilians, five of them children, on 28 June 2017 CNES/Airbus DS "War of Annihilation" – devastating toll on civilians in Raqqa, Syria Amal Othman, 13, and her brothers Ammar, 8 and Mahmoud, 17; and Jamal Aswad, 41 – four of the eight victims killed in a Coalition air strike on 28 June 2017 in Raqqa Amnesty International "War of Annihilation" – devastating toll on civilians in Raqqa, Syria Low-income district west of the Jezra intersection on the western outskirts of Raqqa city destroyed by the Coalition shelling started in June 2017 Amnesty International "War of Annihilation" – devastating toll on civilians in Raqqa, Syria Satellite image showing the houses where 16 members of the Fayad family and neighbours were killed in Coalition strikes on 12 October 2017, before the strike CNES/Airbus DS "War of Annihilation" – devastating toll on civilians in Raqqa, Syria Satellite image showing the houses where 16 members of the Fayad family and neighbours were killed in Coalition strikes on 12 October 2017, after the strike CNES/Airbus DS "War of Annihilation" – devastating toll on civilians in Raqqa, Syria Ali Hassan Nafa; Mahdia Hashish and her husband Hussein Ali; Hussein Hashish, Mohammed Hashish and Hussein Ibrahim Hashish, and Mohammed Ali Hashish Amnesty International "War of Annihilation" – devastating toll on civilians in Raqqa, Syria Harat al Badu, a built up area in central Raqqa, next to the stadium. It was the scene of the final battle in the city between SDF/Coalition forces and IS Amnesty International "War of Annihilation" – devastating toll on civilians in Raqqa, Syria Map showing where the Badran, Hashish, Aswad and Fayab families were living before the coalition air strikes Unosat /Amnesty International

Israel has grown nervous of Iran's growing presence in Syria and threatened to prevent a build-up of pro-Iranian forces near its frontiers with Syria.

Russia, another Syria ally, and Damascus have also said that Western countries are preparing to carry out strikes against Syria ahead of the Idlib offensive. They claim such threats were part of the West's attempt to undermine Syria's drive to restore control over all its territories.

The US and France have warned an Idlib offensive would trigger a humanitarian crisis and that a chemical attack in Idlib would prompt a western retaliation.

In the statement from the Syrian president's office, Mr Assad and Mr Zarif said that resorting to "threats and pressure reflect the failure of those countries to realise their plans for the region after Syria and Iran confronted them".