The US, France and Britain, 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.
Turkey, too, is expressing grave concern about an attack, anxious it will bear the brunt of the humanitarian and security fallout.
Turkish Defence Minister Hulusi Akar held a closed-door meeting with US Special Envoy to Syria, James Jeffrey, in capital Ankara on Tuesday.
Washington sees a potential attack on Idlib as an escalation of an already risky conflict, U.S. secretary says.
The US Defense Department told Sputnik Thursday that it was aware of reports of a huge military exercise Russian Federation planned to stage in the Mediterranean Sea this week, and hoped Moscow would follow the rules of safe conduct.
In this December 20, 2012, file photo, Syrian refugees, who fled their home in Idlib due to a government airstrike, look out of a vehicle's window just after crossing the border from Syria to Turkey in Cilvegozu, Turkey.
"The US sees this as an escalation of an already unsafe conflict", Pompeo said in a post on Twitter in which he also blasted Russian Foreign Minister Sergei Lavrov for "defending (the) Syrian and Russian assault".
Idlib is adjacent to Latakia province, the coastal heartland of Assad's Alawite minority and home to the Russian military airport of Hmeimim.
The International Crisis Group, a Brussels-based think tank, said an all-out assault on Idlib and its catastrophic consequences could still be avoided. In addition, Russian experts are among the Syrians.
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HTS has controlled much of Idlib since 2015, acting as de facto governmental authority, facilitating trade across the long border with Turkey and organising aid deliveries.
It was not immediately clear whether Turkey's decision to update its list of terror groups to include HTS could indicate a green light from Ankara for a possible Russian-backed regime operation into Idlib.
Assad's forces have already begun massing in the areas of Idlib already under its control and on its southern and eastern flanks. The province contains an array of militant groups, including the ultra-radical Levant Liberation Committee (LLC), which is also known as the al-Qaida-linked Nusra Front.
A total of 391 civilians, including 66 children, were killed in war-torn Syria last month, according to a Syrian NGO on Saturday.
Russian President Vladimir Putin will attend a three-way summit in Tehran on September, 7, with the leaders of Turkey and Iran, his spokesman said on Friday.
"We are between two sides. the end of the world that is the border with Turkey and the enemy, or hell, which is coming to burn everything", he said in a series of voice messages sent to The Associated Press.
United Nations officials estimate an offensive would trigger a wave of displacement that could uproot up to 800,000 people.
The terrorists have said that there will be no dialogue with the Russians and the Syrian government to give up Idlib and its suburbs.