Baghdad (AFP) - Facing a string of defeats in Syria and Iraq, the Islamic State group is being forced to retreat to the desert from which it emerged three years ago.

By the end of 2014, the group born in Iraq held one third of the oil-rich country and large swathes of territory in neighbouring Syria.

But today it has lost 90 percent of its territory in Iraq, including the city of Mosul, while in Syria a US-backed alliance of Kurdish and Arab fighters has captured over 60 percent of its one-time bastion of Raqa.

Syrian government troops meanwhile are eating away at the last province under jihadist control, Deir Ezzor, where they broke an IS siege on the provincial capital on Tuesday.

At one time, the group held around half of Syria, much of it uninhabited desert, but today it controls just 15 percent, according to Syria specialist Fabrice Balanche.

Syria's government meanwhile has rapidly recovered ground lost to rebel groups and jihadists and now claims 50 percent of the ravaged country.

Kurdish forces hold around 23 percent, according to Balanche.

In Iraq and Syria, "the Islamic State's governance project (is) compromised, but I don't see the Islamic State completely defeated," said Ludovico Carlino, a senior analyst at IHS Markit Country Risk.

"From a narrative/propaganda perspective, losing Raqa will have surely big implications," particularly after the fall of Mosul, he said.

But he said the Euphrates River Valley, an area of desert stretching from Deir Ezzor province in eastern Syria to Al-Qaim in western Iraq, "from a strategic perspective... is much more important."

- New tactics -

"This is the area where IS will go underground and use as a launchpad for their insurgency."

Commanders in the US-led coalition against IS estimate between 5,000 and 10,000 jihadist fighters and commanders have already fled Raqa to the area.

"The group has basically transferred all its administrative institutions and assets" there, said Carlino.

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The region includes key oil fields, an increasingly precious resource for IS, which has seen its finances decline enormously from their peak, with both oil revenue and tax collection down.

IS faces attack from several fronts and forces in the area, including the US-backed Syrian Democratic Forces (SDF), Syria's army backed by Russia, and Iraq's army.

The jihadists have begun to dig tunnels, plant explosive devices and prepare vehicle bombs, according to the US-led coalition.

"The loss of Raqa is already happening. It is the complete recapture of Deir Ezzor by the Syrian army that will be the real turning point," said Balanche.

Inside IS-held parts of the province, that possibility has created new restrictions and tension, according to activists.

"They've built military barriers in each neighbourhood and alleyway. They've mined the administrative borders to the cities," said Omar Abu Leila, an activist from Deir Ezzor 24, which publishes news on the city.

- Lacking food and water -

The group has also stepped up patrols, inspecting ID cards of local residents and arresting young men, he told AFP.

"IS has planted more spies recently, fearing being compromised just before this expected battle," he added.

As the prospect of IS being driven completely from Syria and Iraq nears, attention is turning to what might follow, and in particular the question of relations between minority and majority groups in the two countries.

The SDF has brought together Kurdish and Arab fighters, but it remains to be seen whether the alliance will withstand Kurdish dreams of federalism.

And it is unclear whether the Syrian government of President Bashar al-Assad will allow other forces to control parts of the country it has spent six years trying to clear of rebels and jihadists.

In Deir Ezzor, civilians in IS-held territory face shortages of food, water and electricity, and are increasingly afraid as the battle approaches, said Abu Leila.

"They are afraid that the SDF will collude with regime forces and hand over the territory from which Daesh was ousted to Assad's forces," he said, using the Arabic acronym for IS.

They also fear revenge attacks by government forces or allies, he said.

IS mined ethnic and sectarian divisions in both Syria and Iraq to recruit members to its cause, and experts warned the group would profit in the absence of real efforts at reconciliation.

And the group will not disappear entirely, said Balanche.

"IS will return to the underground. It will carry out terrorist attacks," he said.

"Some will leave to pursue jihad elsewhere."