The head of Syria's former Al Qaeda affiliate in a video released Sunday urged supporters to "take up weapons" to defend his group's bastion of Idlib against increased bombardment.
The Damascus regime and its Russian ally have in recent weeks upped shelling and air strikes on the northwestern region held by Hayat Tahrir Al Sham (HTS), despite a Russian-Turkish buffer zone deal intended to protect it.
"We call for anybody able to take up weapons... to head to the battlefield," HTS chief Abu Mohammad Al Jolani said.
The spike in violence against the Idlib region since late April signalled "the death of all previous agreements and conferences", he said during an interview with a local media activist in a countryside setting.
It shows that "only jihadists and military might can be relied on", said Mr Jolani, wearing a camouflage uniform and sitting cross-legged in the grass.
The activist said the interview was recorded in the northern Hama countryside, although it was not immediately possible to verify this.
It appeared to have been made in recent days after a regime advance.
Idlib, a region of some three million people, is supposed to be protected from a massive regime assault by a September buffer zone deal signed by Russia and rebel backer Turkey.
But that agreement was never fully implemented after militants refused to withdraw from the planned buffer area.
Damascus and its allies have increased their deadly bombardment of the southern part of the militant bastion in recent weeks, killing dozens of civilians and causing tens of thousands more to flee.
They have taken control of several towns from the militants in recent days.
The uptick in air strikes and shelling has displaced 180,000 people between April 29 and May 9, the United Nations says.
The Syrian Observatory for Human Rights, a Britain-based monitor, says around 120 civilians have been killed in the bombardment.
Damascus has not announced a wide offensive, but instead regularly announces targeting "terrorist" – meaning militant or rebel – positions.
Analysts believe the offensive will be limited.
The regime and Russia have accused the militants of targeting the main Russian airbase of Hmeimim to the west of Idlib.
"It's the right of the revolutionaries to bombard this base which causes the death of... women and children," Mr Jolani said.
"If the Russian leadership wants the bombardment on Hmeimim to stop, it simply needs to stop supporting the regime and killing the Syrian people and civilians," he added.
Russia intervened in the war in 2015, and has since helped the government to regain large parts of the country.
The civil war in Syria has killed more than 370,000 people and displaced millions since it started with the brutal repression of anti-government protests in 2011.