A deadly feud is ripping through Al Qaeda's leadership in Syria as the once-feared terror group looks to position itself as a "moderate" alternative to the disintegrating so-called Islamic State and President Bashar Al-Assad's regime forces.

Key points: At least four high-profile rebel leaders have been assassinated in Idlib in the past month

At least four high-profile rebel leaders have been assassinated in Idlib in the past month Dozens of key Al Qaeda members have been detained

Dozens of key Al Qaeda members have been detained Many fear full-blown conflict will erupt between al Qaeda factions

A group of Al Qaeda-linked factions that hold the province of Idlib are on the brink of all-out war, with a series of assassinations and detentions of high-profile leaders.

The bloody feud pits hardline foreign fighters loyal to Al Qaeda's leader, Ayman al-Zawahri, against more moderate Syrian members loyal to Hay'at Tahrir al Sham (HTS) leader Abu Mohammed al-Julani.

These fault lines within Al Qaeda date back to the early years of the Syrian conflict when Al Qaeda first emerged in the form of Jabhat al Nusra or the Nusra Front.

In 2013, the groups most extreme elements broke away to form the core of Islamic State, or ISIS.

The Nusra Front has since formed and broken alliances, merged and clashed with other extremist groups and twice changed its name.

It is now the main force behind HTS, which had been on the ascendant until recently, but the group's current relationship with Al Qaeda is murky.

Trouble brews in Idlib

Idlib province is home to thousands of fighters, locals civilians and tens of thousands of displaced families who have fled the fighting elsewhere.

Tensions among Islamic fighters that control the province are at breaking point following a series of assassinations and detentions of high-profile Al Qaeda members over the past month.

Several of HTS's most high-ranking leaders have been assassinated under mysterious circumstances.

A rare photo of Tahrir al-Sham leader Abu Mohammad al-Julani released by the group last year. ( Supplied )

Provincial leaders Mounir al-Muslimi and Abu Mahmoud al-Halabi were ambushed and shot by masked assailants while driving inside territory held by HTS on November 30 and December 10 respectively.

Al-Muslimi was known as a master planner of IED and assassination attacks.

Last week, hardline HTS military leader Abu Bakr Ras al-Hosn — known locally for his cruelty, arbitrary arrests and murders — was kidnapped and found dead from multiple gunshot wounds.

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Suspicions of an internal housekeeping from both sides are rife, but there has been no evidence or charges so far.

Late last month, a wave of internal raids and detentions were ordered by Mr al-Julani.

Among those detained were two of Al Qaeda's most esteemed leaders and founding members — Jordanian citizens Sami Oraidi, Al Qaeda's former top religious figure in Syria, and the highly-secretive former military commander in southern Syria, Ayad Toubasi, also known as Abu Julaybib al-Urduni, brother-in-law of the late Al Qaeda in Iraq leader, Abu Musab al-Zarqawi.

They were set free days later after pressure by factions within the group who threatened to withdraw from the battlefield in protest.

During their detentions, other Al Qaeda officials, including a prominent cleric, went to ask Mr al-Julani why their comrades were detained.

But the men were taken into custody themselves.

Al Qaeda backlash

In response, Al Qaeda leader Mr al-Zawahri released an audio recording in which he accused HTS and Mr al-Julani of "betraying the vow of allegiance".

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In Syria, Al Qaeda leader Abu Humam al-Shami warned: "The campaign of arrests against our brothers and cadres will not stop us from continuing our struggle against the enemies of God and will open for you a door that you will wish you never opened."

Days after Mr al-Shami's warning, intense clashes broke out between HTS and the Jund al-Malahem faction that split from it in October and is close to the detained Al Qaeda officials, leaving at least seven people dead

"There are widespread concerns of a full-blown confrontation between the two sides," said an opposition activist in northern Syria who lives in areas controlled by HTS.

Speaking by telephone, he asked that his name not be made public for fear of reprisals by the militants.

The activist said that dozens of other members and commanders, including one known as Abu Khadija who used to run Al Qaeda's notorious al-Iqab prison, were also detained.

"It is clear that the jihadist movement in Syria is suffering probably the worst moment of internal fighting since the 2013 schism between ISIS and Jabhat al-Nusra [now HTS]," said Jennifer Cafarella of the Washington-based Institute for the Study of War.

Rebranding a terror group

Abraham Wagner, senior fellow at the Centre for Advanced Studies on Terrorism, told the ABC that Al Qaeda is in the middle of a transition period.

"Al Qaeda, now Al Qaeda 2.0, has gone through three reincarnations," said Mr Wagner, who is also professor of national security studies, intelligence and law at Columbia University, UCLA, and University of Southern California.

Many fear a full-blown conflict could erupt between Islamic groups in the province of Idlib. ( ABC News: Tracey Shelton )

"They now position themselves as moderate between ISIS, as a brutal terrorist organisation, and the Assad Government, who have managed to kill half a million people or so.

"Al Qaeda in Syria have morphed into this middle alternative and it's not clear where it's going to go."

Since establishing Al Qaeda's Syria branch under Abu Bakr al-Baghdadi, then head of the Islamic State of Iraq, Mr al-Julani has always towed a more moderate line.

This is likely what led to the split with al-Baghdadi, who merged his own group with elements of the then Nusra Front to form the more radical Islamic State of Iraq and Syria, or ISIS.

Both operated for some time as branches of the terrorist group in Syria until Mr al-Zawahri also announced a split from the radical offshoot.

In Syria, al-Julani's men describe him as a strong intelligent leader and his leadership as "moderate" and "just".

But many activists and civilians found this "moderate" enforcement of sharia law harsh and oppressive, fleeing to Turkey or Europe to escape the terrorist group take over.

Still, Mr Wagner said under Mr al-Zawahri, Al Qaeda have rebranded themselves.

"They clearly have toned down," he said.

"They are not interested in massive attacks against the United States now."

Mr Wagner said the group's new vision is now of land and holding a piece of a new state, and if they can resolve their differences, he believes it may be possible in Syria.

"Is Assad going to regain 100 per cent of Syria? Probably not," he said.

"Is he going to go entirely? Probably not," he added, saying that a peace deal in Syria could mean giving autonomous rule to a new moderate Al Qaeda in Idlib.

ABC/AP