ISIS has been forced to reorganise and change strategy to survive after a string of devastating defeats four years after announcing its cross-border 'caliphate' in Iraq and Syria.

The jihadist group has been pinned down to its last desert holdouts in Syria having lost all urban centres previously under its control in neighbouring Iraq.

It has now changed its administrative structure and shifted its focus away from operating the state-like apparatus it once ran.

An Iraqi security official said that after losing ground in Iraq and Syria, 'ISIS leadership is now focused on a global vision' modelled on Al-Qaeda.

ISIS has been forced to reorganise and change strategy to survive after a string of devastating defeats four years after announcing its cross-border 'caliphate' in Iraq and Syria. ISIS chief Abu Bakr al-Baghdadi addresses worshippers at a mosque in the Iraqi city of Mosul in 2014

Its cross-border state now destroyed, ISIS is instead likely to focus on spreading shock and terror around the world through dramatic attacks, the official added.

ISIS will have to find 'a new way of doing things, especially to recruit after heavy losses', said the official, who asked to remain anonymous, told AFP.

At its peak, the self-proclaimed caliphate included 35 'wilaya' (provinces) mostly set within a swathe of territory spanning either side of the border between Syria and Iraq.

But following major military defeats - including the jihadists' loss of their de facto capitals of Raqqa in Syria and Mosul in Iraq - ISIS propaganda outlets now only mention six 'wilaya'.

Former ISIS provinces like Mosul, Raqqa and Kirkuk - an oil-rich province in Iraq - no longer exist.

Instead, the term 'wilaya' is now used to refer to large chunks of territory like Iraq and Syria, along with Somalia, East Asia, Tajikistan and the Egyptian Sinai.

The administrative reshuffle marks a clear switch from 2014, when ISIS leader Abu Bakr al-Baghdadi boasted of having erased the 'imperialist' design that divided the Middle East.

The proclamation was made with great fanfare as jihadists drove bulldozers across the Syrian-Iraqi border, symbolically destroying one of the frontiers drawn up by colonial powers as they carved out the modern Middle East from the ruins of the Ottoman Empire after World War I.

After years battling ISIS, Iraqi troops are now redeployed along most of the border with Syria, across which jihadists and weapons have long flowed unimpeded.

On the Syrian side, separate offensives by forces loyal to Syrian President Bashar al-Assad and a US-backed coalition have pushed ISIS jihadists out of most of the territory they once controlled.

'The change proves Daesh's weakness and the loss of much of its leadership,' the security official told AFP, using an Arabic acronym for ISIS.

ISIS's restructure 'shows its central command lacks confidence in its wilaya commanders in Iraq and that it is reducing their powers to one (central) leadership', the official said.

Iraqi authorities regularly announce the arrest or death of ISIS leaders and relatives of Baghdadi, such as his son, who was killed in Syria in July by Russian missiles.

Baghdadi himself was thought to have been killed several times, and the US has offered up a $25 million (21.5 million euro) reward for information leading to his capture or death.

The jihadist group has been pinned down to its last desert holdouts in Syria having lost all urban centres previously under its control in neighbouring Iraq

In a purported new audio message released on Wednesday to mark the Muslim holiday of Eid al-Adha, Baghdadi called on his followers to 'not give up the jihad against their enemy'.

'Baghdadi's speech was one of consolation and condolence,' said Hisham al-Hashemi, an expert on radical Islamist groups.

It was an 'acknowledgement of defeat... but (Baghdadi) urged those remaining to persevere', he said.

In a first, Baghdadi used his 55-minute recording to call for attacks in the West, saying an operation there would be 'worth a thousand' at home.

Much of the address was reminiscent of approaches long used by Al-Qaeda, according to Hashemi.

In it, the ISIS leader scorns the US, blasts Shiite Iran, and calls on Sunni Muslims in Iraq to denounce the Shiite-dominated paramilitary units of the Hashed al-Shaabi.

Tore Hamming, a jihadist specialist at the European University Institute, said Baghdadi's speech falls into the same category as his last three - 'crisis management'.