The US has said there is significant evidence that chemical weapons are being prepared by Syrian government forces in Idlib, as Russia, Iran and Turkey meet in Tehran to decide the fate of the region.

Turkey’s president, Recep Tayyip Erdoğan, is pressing Russia to order the Syrian regime to back off from an assault on the final large opposition enclave, saying it could lead to a bloodbath or mass influx of refugees into Turkey and perhaps elsewhere in Europe.

Erdoğan, who proposes a plan whereby rebel groups are given the chance to leave Idlib, is meeting Vladimir Putin and Iran’s Hassan Rouhani in the Iranian capital. Although the three leaders are also discussing plans for reconstruction, the return of refugees and the fate of Kurds in Syria, the focus is Russia’s determination to oversee a capture of Idlib by force.

“The illegal presence and interference of America in Syria which has led to the continuation of insecurity in that country, must end quickly,” Rouhani said at the beginning of the meeting.

The UN special envoy for Syria, Staffan de Mistura, is also urging Russia to hold off and to return to talks in Geneva that he has prepared for next week on a future constitution for Syria.

The warning of evidence of an imminent chemical attack was made by the new US envoy for Syria, Jim Jeffrey. The US has repeatedly said it will 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.”

Syria’s president, Bashar al-Assad, has massed his army and allied forces on the frontlines in the north-west, and Russian planes have joined his bombardment of rebels there – the prelude to a possible assault.

Jeffrey said: “I think the last chapter of the Idlib story has not been written. The Turks are trying to find a way out. The Turks have shown a great deal of resistance to an attack.”

He said the US had repeatedly asked Russia whether Washington could “operate” in Idlib to eliminate the last holdouts of jihadist groups. Asked whether that would include US air strikes, Jeffrey said: “That would be one way.”

Turkey has established 12 observation posts around Idlib – an area of 6,000 sq km that is home to about 3 million people – as part of a de-escalation agreement it negotiated with Iran and Russia in the so-called Astana process. During the Tehran meeting Russia will seek to persuade Turkey to withdraw its troops.

Russia and Syria justify the proposed attack on the basis that a large number of jihadists fighters, mainly from Hayat Tahrir al-Sham, operate in Idlib. HTS and its satellites are deemed a terrorist organisation by the UN and as such are not party to any ceasefire.

Large numbers of militants have been bussed to Idlib with the Syrian government’s permission following the collapse of previous rebel enclaves such as Homs, Aleppo and eastern Ghouta.

Some of these groups are openly hostile to Turkey, making the efforts of Erdoğan to persuade these groups to leave that much harder. The absence of an obvious destination further complicates the task. De Mistura has suggested up to 10,000 fighters sympathetic to HTS operate in Idlib.

Turkey is pleading with Russia to put the interests of millions of civilians ahead of a precipitous effort to drive out the militants, most of whom are integrated in the city. The so-called marbling effect, the blending of jihadists with civilians, has been a repeated problem in Syria.

On Thursday, the 10 elected members of the UN security council issued a joint statement calling for restraint, but a debate on the issue at the security council on Friday is likely only to air differences rather than provide a solution.

Olof Skoog, the UN permanent representative of Sweden, warned of a possible humanitarian catastrophe in Idlib.

The US ambassador to the UN, Nikki Haley, has set a strict red line on Russian operations in Idlib, saying on Tuesday that if the Syrian government wanted to “continue to the route of taking over Syria, they can do that but they cannot do it with chemical weapons”.

Russia has built up its fleet in the Mediterranean to counter any possible US assault on the Syrian army.