The Syrian regime and its Russian ally are threatening an offensive to retake the northwestern province of Idlib, Syria's last rebel bastion, where jihadist group Hayat Tahrir al-Sham holds sway.
The jihadist alliance, the core of which is formed by the former Al-Qaeda branch in Syria, is likely to be the regime's toughest foe.
Here is some background.
- What is HTS? -
HTS first appeared in Syria in January 2012 as the Al-Nusra Front, and Syrian President Bashar al-Assad and his Russian ally still refer to the jihadist group by that name.
Classified as a "terrorist" group by the United States, the European Union and the United Nations, it arrived in Syria as an extension of Al-Qaeda in Iraq.
The group's current leader, a Syrian who uses the nom de guerre Abu Mohammad al-Jolani, is a veteran of fighting in Iraq.
In 2013, the group swore allegiance to Al-Qaeda before splitting with the global jihadist syndicate in July 2016 and renaming itself the Fatah al-Sham Front.
In 2017, it dissolved that group to form the backbone of Hayat Tahrir al-Sham.
The group mainly consists of Syrian jihadists, estimated at about 30,000 fighters, according to the Syrian Observatory for Human Rights.
The jihadists are "well organised and battle-hardened", said Syria expert Fabrice Balanche.
"HTS definitely retains a sizeable foreign fighter component, perhaps comprising at least 20 percent of its total fighting force," said Charles Lister, an analyst at the Middle East Institute.
The fighters are mostly from the Middle East, "but also from Russian-speaking areas, Europe and south Asia," he added.
- How powerful is it? -
HTS now controls nearly 60 percent of Idlib province.
It has set up a civil administration that collects customs duties at the border with Turkey and imposes taxes on traders.
The group "derives so much of its power from being the authority over how trade flows into and out of Idlib, which helps fund the group and gives it power beyond its size," said Nicholas Heras, a researcher at the Center for New American Security.
Previously, HTS had a presence in many of the country's rebel-held areas, especially near Damascus and in the south. But it has lost that territory as its fighters were evacuated to Idlib in surrender deals.
HTS has consistently been excluded from ceasefires negotiated by the United Nations or Russia.
The jihadist alliance has been the target of air raids from both Moscow and the US-led anti-jihadist coalition, which have killed several of its senior commanders.
- Who are its rivals? -
Formerly associated with influential Islamist rebel groups like Ahrar al-Sham and Nureddine al-Zinki, HTS underwent a bloody period of power struggles in 2017 that included battles with former allies, creating resentment that persists today.
In early 2018, Ahrar al-Sham and Nureddine al-Zinki announced their Turkey-backed merger to counter the growing power of HTS.
They joined four other rebel factions in early August to form a new coalition -- the National Liberation Front.
Separately, HTS has increased raids in recent weeks against "sleeper cells" linked to the Islamic State group (IS), which has claimed responsibility for assassinations and bombings targeting HTS's leaders and fighters.
The two jihadist heavyweights have also clashed in Syria's northern city of Raqqa and in eastern Deir Ezzor province.
In July 2014, Nusra's chief said the group's goal was to set up an "Islamic emirate", akin to the "caliphate" proclaimed by IS shortly beforehand.
- What is its future? -
On August 22 the reclusive Jolani broke a long silence to reiterate that the group was determined to repel any offensive by Damascus.
"Just thinking about surrendering to the enemy and handing over weapons is an act of treason," he said.
Russia has called for the dissolution of HTS, but neighbouring Turkey is trying to negotiate a solution with the jihadists to avoid a large scale offensive that would destabilise the border area, the Britain-based Observatory said.
According to Heras, the jihadist alliance's "dissolution on the command of Turkey would rob it of much of its power, which would replace HTS rule with Turkish rule."