While Russia and Syria in the past have often have referred to all of their armed opponents as "terrorists", in the case of Idlib, they appear to be making a distinction in a bow to Turkey, which provides backing for numerous more moderate Sunni rebels in the province and which has joined Moscow and Tehran in sponsoring Syrian peace negotiations in the past year. Tens of thousands were bused there as part of surrender deals with the government. Ankara fears a humanitarian and security crisis on its borders.
Idlib, the Syrian opposition's only remaining stronghold in the country and now a refuge for over a million displaced Syrians, is likely to be the last major theater of battle after seven years of brutal civil war.
The US well aware that any possible military action against the legitimate government of Syria must be made by consensus of the worldwide community. Thousands protested in various towns in Idlib and Hama, denouncing threats of an attack and hailing the area's readiness to fight. But it would hardly signal the end of the conflict or its miseries. But the strongest alliance of fighters is led by the LLC that controls most of the area.
The province of Idlib, northwest of Syria and very close to the Turkish border, is partly (some 60 percent, according to intelligence estimates) under the control of terrorist and extremist groups.
The situation in Idlib is complex as it has emerged as the main destination for the rebels who surrendered in many areas across Syria. More than 350,000 people have been killed, and more than 11 million have fled their homes. Russian Federation has said a military operation there is necessary to weed out "terrorists" it blames for attacking its bases on the coast. "The 3 million Syrians who have already been forced out of their homes and are now in Idlib will suffer from this aggression", he wrote in a Twitter post on Friday. "Not good. The world is watching", Pompeo said.
The American warning follows 10 days of toughly worded exchanges between Western powers on the one side, and Syria and its Russian ally on the other, just as Assad - bolstered by crucial Russian air support since 2015 - sets his sites on Idlib.
"Sergey Lavrov is defending Syrian and Russian assault on #Idlib", Pompeo tweeted on Friday.
"They will also underscore that the United States will respond to any chemical weapons attack perpetrated by the Syrian regime", the State Department said. In addition, Russian experts are among the Syrians. It has been argued that Turkey may be involved in convincing Tahrir al-Sham (called also Jabhat al-Nusra) to merge with other armed groups. HTS is now the most powerful armed faction in Idlib.
Assad's forces have already begun massing in the areas of Idlib already under its control and on its southern and eastern flanks.
Over the weekend, Russian Federation began a naval exercise in the Mediterranean, just a few hundred miles from the likely front lines, involving 26 warships and support vessels, as well as 36 planes.
A total of 391 civilians, including 66 children, were killed in war-torn Syria last month, according to a Syrian NGO on Saturday. Lavrov last week described Idlib as a "festering abscess" that needed to be drained.
It took that territory after offensives against the two mutually hostile groups that previously controlled it: Islamic State and the Kurdish YPG militia. 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.
Iranian Foreign Minister Mohammad Javad Zarif visited Damascus to discuss plans for an upcoming summit between the leaders of Iran, Russia and Turkey, which Tehran will host on September 7 to discuss Idlib, Iran's Fars news agency reported.
He claims the number of foreign terrorist fighters could be as high as 15,000.