An all-out assault on the rebel enclave of Idlib in north-west Syria drew closer on Friday after Russia and Iran rejected a Turkish call to back a ceasefire. The move, at a stormy summit in Tehran, left the three countries to paper over their differences through a broadly worded joint statement calling for all sides to back a political settlement to end Syria’s civil war.

The outcome of the summit led to warnings from the west that Russia was pushing Syria towards the edge of an abyss, while US officials claimed it had evidence that the Syrian government was preparing to use chemical weapons.

At a closing press conference, Russia’s Vladimir Putin and Iran’s Hassan Rouhani openly clashed with Turkish president Recep Tayyip Erdoğan. Putin insisted that the removal of terrorists from Idlib was the first priority.

Facebook Twitter Pinterest The official memorial plate of the Trilateral summit on Syria including Putin, Rouhani and Erdoğan Photograph: Handout

Erdoğan at the last minute urged his fellow leaders to include a ceasefire in the text of the final communique, but Putin said the armed opposition groups, including those designated terrorists, were not present at the talks, so no such commitment could be made.

It is unclear whether Russia will now order Syrian government forces to pause in the face of Erdoğan’s resistance, and growing warnings from the west of a humanitarian disaster if a full-scale military escalation goes ahead.

The UN special envoy for Syria, Staffan de Mistura, urged Putin to allow more time for efforts to separate UN-designated terrorists from within the the population of Idlib. de Mistura said this was feasible if Turkey was given time to mobilise civilians in Idlib to send a message to the fighters to evacuate the towns and villages. He held up candles to symbolise the wishes of the civilian population.

At the Tehran summit Erdoğan warned: “Any assault, whatever the reason for it, will inevitably lead to a catastrophe, killings and a major human tragedy. We never want to see Idlib to turn into a lake of blood. We have to find a rational way out in Idlib that could meet our security concerns.” He added that millions of refugees would flee towards Turkey if a major attack against rebels took place.

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As many as 3.5 million people live in the Idlib region, the final enclave holding out against the regime of Syrian president, Bashar al-Assad. Almost half of them have been displaced from other towns recaptured by the Syrian government, such as Homs, Aleppo and Ghouta, and have nowhere left to escape within Syria.

Erdoğan explained that Turkey had neither “the strength or capability” to host a fresh influx of refugees, raising the prospect that refugees would head to Europe.

But Putin insisted that the UN-designated jihadist terrorists inside Idlib had to be removed, and implied that Turkey had already tried and failed to negotiate their voluntary removal to separate them from legitimate opposition groups. Putin said Russia found it “unacceptable” when civilians were used a pretext to “shield terrorists” and target Syrian government positions.

He added: “We hope that representatives of terrorist organisations will have enough common sense to stop resistance and lay down their weapons.”

Putin’s rejection of a ceasefire was backed by Rouhani, who insisted that there could be no compromise. “It is necessary for the fight to continue until all terrorist groups in Syria, especially in Idlib, are eradicated.”

The Russian stance led to condemnation by western diplomats at an emergency meeting of the UN security council in New York. The US ambassador to the UN, Nikki Haley, accused Russia of using the “playbook of death” and said any attack on Idlib would be considered a serious escalation. François Delattre, the French ambassador to the UN, warned that Syria “was at the edge of an abyss”, and said Russia would be held responsible if a massacre occurred.

Facebook Twitter Pinterest Recep Tayyip Erdoğan is welcomed by Iranian minister Mohammad Shariatmadari after touching down in Tehran on Friday. Photograph: Anadolu Agency/Getty Images

The UK ambassador, Karen Pierce, said there were more babies than terrorists in Idlib, and named the Syrian forces preparing to attack the region, promising that they would be held accountable if indiscriminate attacks on civilians went ahead.

John Ging, the UN’s senior humanitarian official, said: “Idlib has the potential to create a humanitarian emergency on a scale not yet seen in this crisis.”

Turkey shares the Russian aim of removing the jihadists, mainly members of Hayat Tahrir al-Sham (HTS), and would like them to hand their weapons over to the Free Syrian Army. However, Russia claims the brutality and breadth of the terrorist groups means they dominate the 80% of the population that want peace.

HTS is designated by the UN as a terrorist group, and has been sending weaponised drones towards Russian positions. De Mistura has estimated that there are 10,000 UN-designated terrorists in the region. Russia puts the figure higher.

Turkish troops are stationed at 12 observation points inside Idlib, over an area of 6,000 sq miles, sent to the region as part of an earlier de-escalation agreement. Turkey will now have to decide how to make sure they do not become collateral damage in the attack.

Earlier, the US said there was significant evidence that chemical weapons were being prepared by Syrian government forces. The warning was made by the new US envoy for Syria, Jim Jeffrey. The US has repeatedly said it would not tolerate a chemical attack, but has not said how it would respond.

Jeffrey said: “I am very sure that we have very, very good grounds to be making these warnings. Any offensive is to us objectionable as a reckless escalation. There is lots of evidence that chemical weapons are being prepared.”

Separately, Russia has warned the US to move a small group of troops out of a base in southern Syria, saying it intends to attack the area where the Al Tanf US airbase is located due to the presence of militants. The Pentagon has ordered its troops to remain at the base.

• The map of Syria in this article was updated on 11 September 2018.