AMMAN (Reuters) - Syrian rebels bracing for battle in northwest Syria are pinning their hopes on ally Turkey to intervene with Russia and prevent an all-out Syrian government offensive that could deal a final blow to their seven-year-long uprising.

Syrian President Bashar al-Assad has massed his army and allied forces on the frontlines in the northwest and on Tuesday Russian planes joined his bombardment of rebels there, the prelude to a possible assault.

The fate of the insurgent stronghold in and around Idlib province now seems to rest on a summit meeting to be held in Tehran on Friday between the leaders of Assad’s supporters Russia and Iran, and the rebels’ ally Turkey.

“We realize the extent of vengeance and massacres that will befall us if they get our heads. They will slaughter us. The coming battle is to be or not to be,” said Mustafa Sejari, a Free Syrian Army (FSA) commander preparing for the offensive.

Backed by Russian air power, Assad has in recent years taken back one rebel enclave after another. Idlib and its surroundings are now the only significant area where they remain in active armed opposition to Damascus.

Rebels backed by Turkey also hold an adjacent zone in Syria along the border between the two countries where Ankara has helped them set up a local administration. But they have not fought Assad from there.

With Idlib standing as the last active redoubt of the rebellion, the insurgents there say they have no choice but to fight to the finish. Previous battles ended with defeated insurgents agreeing surrender deals that involved them being bussed across the country to Idlib.

“There is no other Idlib to move to. We either die in the area or we resist until we win and stay,” said Mohammad Rasheed, another rebel official in Syria.

Both Sejari and Rasheed are from rebel factions that fought under the FSA banner - groups that have long been at odds with the jihadist factions that dominate in large parts of Idlib.

TURKISH ASSURANCES

The most powerful jihadist grouping in Idlib is the Tahrir al-Sham alliance, spearheaded by al Qaeda’s former Syrian affiliate previously known as the Nusra Front.

Rebels from both Tahrir al-Sham and FSA groups said they had set aside their disputes to face the common enemy - the Assad government. The FSA has accelerated training of new recruits to send to frontlines, Rasheed said.

Russia says there is no option in northwest Syria but to root out the Nusra Front, calling the area “a terrorist nest” and “a festering abscess” that must be cleansed.

Turkey - along with the United Nations - also classifies Tahrir al-Sham as a terrorist group, but says any action should distinguish between it and civilians. An attack would cause a massacre, it says.

Turkey agreed with Russia and Iran last year to make Idlib a “de-escalation zone” of reduced conflict, and sent in army units to erect a string of observation posts along the frontlines between rebels and the army.

They now act as a tripwire in any major Syrian government assault, potentially triggering a wider escalation with Turkey if there is no political deal.

Rebels briefed by Ankara on the Russia-Turkey talks on Idlib say it has offered them assurances it is working on a deal to avoid the saturation air strikes that paralyzed everyday life in other enclaves where insurgents were forced to surrender.

They hope Ankara will maintain the arc of territory along the border from Afrin to Jarablus where it has set up a local administration - a potential last refuge inside Syria for Assad’s opponents.

They also hope Turkey’s regional clout, diplomatic skill and ground presence might spare Idlib the fate of enclaves such as those in east Aleppo, eastern Ghouta and Deraa, they said.

“Turkey’s presence will hopefully not allow the scenario of Ghouta or Deraa to repeat itself,” said Abdul Hakim al-Rahmoun, a rebel commander.

LESS HOPEFUL

Other rebels are less hopeful.

Idlib and surrounding areas have borne the brunt of years of Russian and Syrian military air strikes that have killed thousands of civilians.

More than half the area’s 3 million inhabitants are already displaced people who have fled their homes.

Turkey fears an assault will cause a new wave of displacement, adding to the 3.5 million Syrian refugees in its own territory.

But some rebels say Russia is intent on ending all opposition to Assad and will use the presence of Tahrir al-Sham in Idlib as a pretext to push Turkey into accepting a military offensive.

In the meantime they are making preparations - detonating bridges, digging trenches and sending recruits to the frontlines.