Meanwhile, Russia has sent an armada of ships, including a Marshall Ustinov missile cruiser, to the coast of Syria in the Mediterranean, prompting speculation they were preparing for an imminent attack.
Seized from government forces in 2015, Idlib and adjacent areas form the last major chunk of territory still in rebel hands.
But Russia and Iran have insisted that extremist groups in Idlib must be defeated and are expected to back regime forces in any assault.
With some 3 million people now living in Idlib and the surrounding area, United Nations officials have warned that a government offensive could displace 800,000 people.
It follows comments by Iran's foreign minister, Mohammad Javad Zarif, that "terrorists must be purged" from Idlib and the entire northwestern province returned to government control.
Located near the Turkish border, Idlib has been designated as a de-escalation zone where acts of aggression are expressly forbidden as part of the ongoing Astana process.
ICG said Russian Federation, whose air support would be crucial for the offensive to succeed, should understand that a bloodbath in Idlib would jeopardise its own political goals.
"Russia seeks to ensure not just the regime's military victory in Syria but its full political restoration through global re-legitimation at war's end".
At the request of Damascus, Iran has been offering advisory military assistance to the Syrian army.
Britain, France and the U.S., which together launched limited attacks on Syrian installations in mid-April in retaliation for an alleged Syrian chemical attack, say their red line against illegal weapon use remains in force.
"Frankly speaking, I don't think there's anything the United States can do about it. Let's go in. Let's kill the f*cking lot of them", Trump told Secretary of Defense James MattisJames Norman MattisState Department applauds Saudi-led coalition's admission of "mistakes" in deadly Yemen strike Saudi-led coalition says deadly strike on bus in Yemen was unjustified Pentagon canceling 0M in aid to Pakistan, citing record on militants: report MORE, according to a soon-to-be-released book by journalist Bob Woodward.
Behind the scenes, American diplomats have been actively warning Moscow, which has been accused in the past of turning a blind eye to chemical weapon use by its Syrian proteges.
After seven years of war, the northern province of Idlib has become the rebels' final bastion.