Syrian rebels have pinned their hopes on Turkey to intervene with Russia and prevent an all-out offensive which could deal a fatal blow to their seven-year-long uprising.

Forces loyal to Syria’s president, Bashar al-Assad, are massed on the frontlines in the northwest of the country and on Tuesday Russian planes joined in the bombardment of rebels in a prelude to a possible assault.

The insurgent enclave in and around Idlib represents the final major stronghold in the civil war against the Syrian regime.

Its fate now appears to rest on a summit meeting to be held in Tehran on Friday between the leaders of President Assad’s supporters, Russia and Iran, and Turkey, which has backed the rebels.

“We realise 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,” Mustafa Sejari, a commander in the Free Syrian Army (FSA) preparing for the offensive, told Reuters.

It comes as Russia’s Defence Ministry confirmed four of its jets struck targets in Idlib with airstrikes which the UK-based Syrian Observatory for Human Rights said killed 13 civilians but no fighters.

Russia said the jets hit targets linked to an al-Qaida-affiliated group, including a weapons depot and a launch pad for drones that Russia says have targeted its military bases in Syria.

Backed by Russian air power, President Assad has taken back one rebel enclave after another in recent years.

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 President Assad from there.

Fears for 3m people in rebel-held Idlib ahead of regime offensive Show all 13 1 /13 Fears for 3m people in rebel-held Idlib ahead of regime offensive Fears for 3m people in rebel-held Idlib ahead of regime offensive A Syrian protester waves a flag of the opposition as during a protest against the regime and its ally Russia, in the rebel-held town of Maaret al-Numan in the north of Idlib province AFP/Getty Fears for 3m people in rebel-held Idlib ahead of regime offensive Syrian rebel fighters from the recently-formed "National Liberation Front" stand guard over a trench as rebels prepare defensive positions in anticipation for an upcoming government forces offensive AFP/Getty Images Fears for 3m people in rebel-held Idlib ahead of regime offensive Syrians chant slogans and wave flags of the opposition as they protest against the regime and its ally Russia, in the rebel-held town of Maarrat al-Numan in the north of Idlib province on August 31, 2018. - Its hospitals are battered, residents heavily dependent on aid and escape routes to neighbouring Turkey sealed. If attacked by government forces, Syria's rebel-held Idlib is poised for a humanitarian calamity. (Photo by Zein Al RIFAI / AFP)ZEIN AL RIFAI/AFP/Getty Images ZEIN AL RIFAI AFP/Getty Images Fears for 3m people in rebel-held Idlib ahead of regime offensive Syrian rebel fighters pile-up sandbags AFP/Getty Images Fears for 3m people in rebel-held Idlib ahead of regime offensive Syrians chant slogans and wave flags of the opposition as they protest against the regime and its ally Russia, in Binnish in Syria's rebel-held northern Idlib province on August 31, 2018. (Photo by Muhammad HAJ KADOUR / AFP)MUHAMMAD HAJ KADOUR/AFP/Getty Images MUHAMMAD HAJ KADOUR AFP/Getty Images Fears for 3m people in rebel-held Idlib ahead of regime offensive A Syrian rebel fighter looks through an embrasure in a make-shift bunker AFP/Getty Images Fears for 3m people in rebel-held Idlib ahead of regime offensive A Syrian protester chants slogans as he waves a flag of the opposition as he protest against the regime and its ally Russia, in the rebel-held town of Maaret al-Numan in the north of Idlib province on August 31, 2018. - Its hospitals are battered, residents heavily dependent on aid and escape routes to neighbouring Turkey sealed. If attacked by government forces, Syria's rebel-held Idlib is poised for a humanitarian calamity. (Photo by Zein Al RIFAI / AFP)ZEIN AL RIFAI/AFP/Getty Images ZEIN AL RIFAI AFP/Getty Images Fears for 3m people in rebel-held Idlib ahead of regime offensive Syrian rebel fighters walk through a trench AFP/Getty Images Fears for 3m people in rebel-held Idlib ahead of regime offensive A Syrian protester waves a flag of the opposition AFP/Getty Images Fears for 3m people in rebel-held Idlib ahead of regime offensive A Syrian rebel fighter stands guard over a trench AFP/Getty Images Fears for 3m people in rebel-held Idlib ahead of regime offensive A Syrian rebel fighter from the recently-formed "National Liberation Front" walks through a trench as rebels prepare defensive positions in anticipation for an upcoming government forces offensive, in the countryside of the rebel-held northern Idlib province on August 31, 2018. - In recent days, both the government and its ally Russia have stepped up their rhetoric against the rebel presence in Idlib as they gear up for an assault, which could be the last major battle of the civil war that has torn the country apart since 2011. (Photo by OMAR HAJ KADOUR / AFP)OMAR HAJ KADOUR/AFP/Getty Images OMAR HAJ KADOUR AFP/Getty Images Fears for 3m people in rebel-held Idlib ahead of regime offensive Syrians chant slogans and wave flags of the opposition as they protest against the regime and its ally Russia, in Binnish in Syria's rebel-held northern Idlib province on August 31, 2018. (Photo by Muhammad HAJ KADOUR / AFP)MUHAMMAD HAJ KADOUR/AFP/Getty Images MUHAMMAD HAJ KADOUR AFP/Getty Images Fears for 3m people in rebel-held Idlib ahead of regime offensive Syrian rebel fighters from the recently-formed "National Liberation Front" walk through a a trench as they prepare defensive positions in anticipation for an upcoming government forces offensive, in the countryside of the rebel-held northern Idlib province on August 31, 2018. - In recent days, both the government and its ally Russia have stepped up their rhetoric against the rebel presence in Idlib as they gear up for an assault, which could be the last major battle of the civil war that has torn the country apart since 2011. (Photo by OMAR HAJ KADOUR / AFP)OMAR HAJ KADOUR/AFP/Getty Images OMAR HAJ KADOUR AFP/Getty Images

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,” Mohammad Rasheed, another rebel official in Syria, told Reuters.

Both Mr Sejari and Mr 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.

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.

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. It says an attack would cause a massacre.

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

More than half the area’s three 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.