At a trilateral summit in Tehran, Presidents Putin, Rouhani and Erdogan appeared to agree on the need to protect civilians while targeting “terrorists” in Idlib.
Read More: Syria war: Russia rejects Turkey's calls for Idlib truce (BBC News)
Turkey - which has long backed some rebel groups - fears an all-out assault will trigger another major refugee crisis on its southern border.
Turkish President Recep Tayyip Erdogan told his Russian and Iranian counterparts: "We don't want Idlib to turn into a bloodbath."
"Any attack on Idlib would result in a catastrophe. Any fight against terrorists requires methods based on time and patience."
However, Russia and Iran are allied to Syria's President Assad, and say jihadist groups in Idlib must be wiped out.
Russian air strikes, and thousands of fighters backed by Iran, have helped the Syrian military attack rebel areas.
At Friday's meeting, Iran's President Hassan Rouhani argued that "fighting terrorism in Idlib is an unavoidable part of the mission of restoring peace and stability to Syria".
Meanwhile Mr Putin said "the legitimate Syrian government has a right and must eventually take under control of its entire national territory".
The province is the last major stronghold of the rebel and jihadist groups which have been trying to overthrow Mr Assad for the past seven years.
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Up to 30,000 rebel and jihadist fighters are believed to be in Idlib.
But the vast majority of Idlib's residents are civilians. The UN says the region is home to some 2.9 million people, including a million children.
More than half of the civilians have already been displaced at least once from elsewhere in Syria and have nowhere left to go.
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Without a political solution in Syria, "we will see this war reach new levels of horrors", the UN special envoy on Syria, Staffan de Mistura, said.
Meanwhile, the new US envoy for Syria, Jim Jeffrey, said the anticipated conflict would be a "reckless escalation", and that "there is lots of evidence that chemical weapons are being prepared."