The Syrian regime will claw back the country's last major opposition bastion in stages, experts say, chipping away at Idlib until jihadists and rebels have run out of options.

Since December, Russia-backed regime forces have pressed a blistering assault against the Idlib region, retaking town after town despite warnings from rebel ally Turkey to back off.

"The Assad regime does not currently have the manpower or resources to capture all of Idlib in one operation," said Nicholas Heras of the Institute for the Study of War.

Instead, retaking the region dominated by Syria's former Al-Qaeda affiliate will require Russian backing over the course of "multiple operations with limited objectives".

The latest offensive against Idlib, home to some three million people, comes despite several failed ceasefire deals.

The most significant deal was signed by Moscow and Ankara in the Russian town of Sochi in 2018, aiming to avert an all-out government onslaught.

Under that agreement, a buffer zone was to be created to separate jihadist and rebel fighters in the region from pro-Damascus combatants outside it.

In exchange, traffic was to return to two key commercial arteries running across the bastion, linking the capital and coast to second city Aleppo.

But tensions saw the deal sidelined, and the regime gradually chipped its way northwards to claim towns on those two highways.

On Saturday its forces took control of the strategic crossroads town of Saraqeb in the latest gain of a weeks-long offensive.

North of those roads lie some 50,000 combatants - more than half of them rebels and the rest jihadists, according to estimates from the Syrian Observatory for Human Rights war monitor.

Battles to take those areas would be costlier than either Moscow or Damascus would want to bear at this point, Heras said.

Turkey has called on the regime to halt its campaign. It already hosts some 3.7 million refugees and fears a new influx.

Since December, the violence has killed more than 300 civilians and sent more than 580,000 people fleeing towards the Turkish border.

Tensions flared on Monday after a rare escalation between Syrian regime and Turkish forces in Idlib killed more than 20 people on both sides.

Turkey has since sent additional troops into northwest Syria, causing Damascus to accuse Ankara of trying to hamper its advance by their presence.

Syria analyst Samuel Ramani said further regime progress "depends on whether the Syrian army faces stiffened resistance from Turkish forces or Turkish retaliatory strikes".

If it did, Syrian President Bashar al-Assad "might want to freeze the conflict temporarily and then re-escalate once this crisis in Syria-Turkey relations blows over", he added.

A fresh demilitarised line could be established protect civilians.

"A new buffer zone in Idlib is probably the most likely option, but it's unclear whether Turkey would ultimately see that through," he said.

Assad, whose forces now control around 70 percent of Syria, has repeatedly vowed to retake the entire country.

Waddah Abd Rabbo, editor of pro-government daily Al-Watan, told AFP the reconquest of Idlib would happen "in stages, according to international agreements such as Sochi".

After "the highways are re-opened, it will be imperative to move forwards and liberate the whole province".

Until then, he added, civilians in Idlib would have the option of settling their status with the regime and Syrian fighters could surrender.

Foreign fighters in Idlib are estimated in their thousands and have in the past hailed from Uzbekistan, Chechnya and China's ethnic Uighur minority.

"They will have no choice but to surrender and face trial by the Syrian judiciary, or flee back to where they came from - that is, Turkey," Abd Rabbo said.

Idlib's population has swelled in recent years, mostly with civilians but also with thousands of fighters evacuated under surrender deals from other parts of the country.

If Idlib were retaken, those local fighters would have few options left.

Ramani said the fate of Idlib's rebels largely "depends on Turkey's future course of action".

Late last month, French President Emmanuel Macron accused his Turkish counterpart of sending ships laden with Syrian mercenaries to fight in the Libyan conflict.

"Some Syrians will join Turkey as foreign fighters in Libya," Ramani said.

"But for most, they are at the mercy of Assad's forces" or face life as refugees.