Iran and Russia have backed a military campaign to retake the last rebel-held stronghold in Syria as Turkey pleaded for a ceasefire.
The disagreement narrows the chances of a diplomatic solution to avoid what many say would be a bloody humanitarian disaster.
The trilateral summit in Tehran involves Iranian President Hassan Rouhani, Russian President Vladimir Putin and Turkish President Recep Tayyip Erdogan.
It puts further pressure on the rebel forces still operating in Syria’s northwestern Idlib province, including about 10,000 hardcore jihadists and al Qaida-linked fighters.
It left the chance, however slim, for further diplomacy to try to separate civilians and rebels from the Islamic militants in Idlib.
While Mr Putin called for the “total annihilation of terrorists in Syria”, he left open the possibility of a ceasefire. Mr Rouhani also spoke of “cleansing the Idlib region of terrorists”, while also noting the need of protecting civilians.
Turkey, which backed opposition forces against Syrian President Bashar Assad, fears a military offensive will touch off a flood of refugees and destabilise areas it now holds in Syria. Ankara also has hundreds of troops manning 12 observation posts in Idlib.
“Idlib isn’t just important for Syria’s future; it is of importance for our national security and for the future of the region,” Mr Erdogan said.
“Any attack on Idlib would result in a catastrophe. Any fight against terrorists requires methods based on time and patience. We don’t want Idlib to turn into a bloodbath.”
Mr Erdogan also sought to use Persian literature to drive home his point in Tehran, quoting the poet Saadi: “If you’ve no sympathy for human pain, the name of a human you cannot retain.”
The US also warned against an assault in Idlib, with Ambassador Nikki Haley telling the UN Security Council that “the consequences will be dire”.
North-western Idlib province and surrounding areas are home to about 3 million people — nearly half of them civilians displaced from other parts of Syria.
For Russia and Iran, both allies of the Syrian government, retaking Idlib is crucial to complete what they see as a military victory in Syria’s civil war after Syrian troops recaptured nearly all other major towns and cities, largely defeating the rebellion against Assad.
A bloody offensive that creates a massive wave of death and displacement, however, runs counter to their narrative that the situation in Syria is normalising, and could hurt Russia’s longer-term efforts to encourage the return of refugees and get Western countries to invest in Syria’s post-war reconstruction.
Russia also wants to maintain its regional presence to fill the vacuum left by the US and its long uncertainty over what it wants in the conflict.
“We think it’s unacceptable when (someone) is trying to shield the terrorists under the pretext of protecting civilians as well as causing damage to Syrian government troops,” Mr Putin said.
“As far as we can see, this is also the goal of the attempts to stage chemical weapons incidents by Syrian authorities. We have irrefutable evidence that militants are preparing such operations, such provocations.”
Mr Putin offered no evidence to back his claim. The UN and Western countries have blamed Assad’s forces for chemical weapons attacks in the civil war, something denied by Russia and Syria. The US, Britain and France have vowed to take action against any further chemical attacks by Assad’s regime.
Reacting to Mr Erdogan’s proposal for a ceasefire in Idlib, Mr Putin said “a ceasefire would be good” but indicated that Moscow does not think it will hold.
“We hope that we will be able to reach an agreement and that our call for reconciliation in the Idlib area will be heard,” the Russian president said. “We hope that the representatives of those terrorist organisations will be smart enough to stop the resistance and lay down arms.”
There was no immediate reaction from fighters in Idlib. Naji al-Mustafa, a spokesman for the Turkey-backed National Front for Liberation, said before the summit that his forces were prepared for a battle that they expect will lead to a major humanitarian crisis.
“Idlib is about a lot of international power play and everyone is looking after their interests,” Mr al-Mustafa said.