Aid agencies fear that as many as 700,000 people could be displaced within a day of a Russian-led bombardment of Syria’s Idlib province as momentum towards an expected attack on the last large rebel enclave continues to gather pace.

A three-way summit between Russia, Turkey and Iran to be held in Tehran on Friday has been described by Ankara as a “last chance to avoid a massacre” in north-western Syria, where up to 3 million people live in a densely packed corner of the country.

Their fate is central to the outcome of the seven-year war, in which Idlib city and its surrounding area had increasingly been used to rehome people exiled from elsewhere in Syria who had fled areas won in battle by forces allied to Bashar al-Assad.

Large numbers of people internally displaced in the province depend entirely on aid, with little access to clean water, sanitation or schooling and facing a shortage of food that would almost certainly be exacerbated in the event of an attack.

Donald Trump warned Syria that “the world is watching”.

The US president told reporters in the Oval Office during a meeting with the emir of Kuwait: “That cannot be a slaughter. If it’s a slaughter, the world is going to be very, very angry. And the United States is going to be very angry, too.”

Amnesty International and the Norwegian Refugee Council also called for the planned assault to be abandoned, with both organisations fearing the consequences would be far worse than in other cities attacked earlier in the war.

“The shocking civilian death tolls and war crimes witnessed recently in other parts of Syria such as eastern Aleppo city, eastern Ghouta and Daraa must not be repeated in Idlib,” said Amnesty’s Middle East campaign director Samah Hadid.

“People cannot bear the consequences of yet another offensive using prohibited tactics such as starvation of civilians and indiscriminate bombardment.”

Russian and Syrian rhetoric in the lead-up to the anticipated strike has depicted the area as controlled by extremist groups, particularly the al-Qaida-aligned al-Nusra Front. Opposition groups remain deeply divided, and an extremist alliance, Tahrir al-Sham, holds sway in some areas. Russia has demanded that Turkey use its influence in northern Syria to dissolve Tahrir al-Sham – an outcome that Ankara says it cannot deliver.

A Turkish-backed group of former anti-Assad units has, however, gathered strength in recent months.

Over the past week, military officials have sent armoured columns to safeguard its proxies, and on Wednesday deployed tanks along stretches of its southern border.

Turkey is yet to offer a safe haven to citizens wanting to flee the expected barrage. Over the past 18 months it has established control of a swath of northern Syria, in which its forces and proxies operate safely. However, officials say they have made no decisions about allowing the area to be used as a humanitarian corridor if sustained airstrikes are launched.

The Norwegian Refugee Council (NRC) said safe passage would be essential in the event of an assault. “A staggering 700,000 people could be displaced overnight,” it said. “Already people are on the move again out of fear of what might come next, and are struggling to find shelter.

“As NRC witnessed in eastern Ghouta or Aleppo, routes to safety came too little too late for thousands of civilians trapped in the violence. The only way to truly ensure the safety of civilians in Idlib is to prevent an outbreak in violence altogether. We must see the world’s top powers commit to peaceful negotiations and agreements which protect civilians and grant them safety.”

Nearby east Aleppo was recaptured from opposition groups by a Russian and Iranian-backed assault in December 2016. Throughout the past two years Russian air power and the battlefield prowess of Shia militias directed by Iran have been instrumental in clawing back control of Syria for Assad.

Erdoğan raises fears of a massacre in Syria as major assault looms Read more

Syrian forces have in many cases played a secondary role in the conflict’s most seminal clashes. However, they have gathered in large numbers south of Idlib, along with Shia militias who have consolidated their presence alongside Kurdish forces who had played a lead role in fighting Islamic State, with staunch US support, in the north-west of the country.

The three-way summit in Tehran does not include Syrian officials – a sign of who is calling the shots at such a decisive phase in the conflict.

Turkey has accepted close to 2 million refugees from Syria; however, its political will to sustain their presence in camps along the border has waned, along with public sympathy among Turks wary of the toll of the long war on the country’s troubled economy.