Syrian troops on Friday retook the last major city where the Islamic State group had a presence as Iraqi forces seized a crossing by the jihadists' last urban bastion across the border.

The simultaneous assaults on Deir Ezzor in eastern Syria and Al-Qaim in western Iraq dealt fresh blows to IS in its former heartland, leaving Albu Kamal, on the Syrian side of the border, the last town of note under its full control.

The jihadist group that once laid claim to a self-styled "caliphate" spanning swathes of Syria and Iraq has seen its proto-state crumble in recent months under the pressure of multiple offensives.

In October, it lost its one-time de facto Syrian capital Raqa after an assault of more than four months waged by the US-backed Syrian Democratic Forces, a Kurdish-Arab alliance.

On Friday, Syria's army announced that its Russian-backed assault had recaptured all of Deir Ezzor city, in the oil-rich east of the country, while Iraqi forces captured the Husaybah border post on the edge of the town of Al-Qaim.

"The army forces... restored security and stability to all of Deir Ezzor city," a spokesman for the army command said in a statement broadcast live on state television.

"Deir Ezzor represents the final phase in the complete elimination of Daesh," the statement added, using the Arabic acronym for the group.

The city "was the headquarters of the organisation's leadership, and in losing it, they lose their capacity to direct terrorist operations," the statement added.

State television said engineering units from the army were combing captured neighbourhoods to clear mines and other explosives.

Syrian forces entered Deir Ezzor city in September, breaking an IS siege of nearly three years on government-held parts of the provincial capital.

The battle has been ferocious, with heavy Russian air strikes and Syrian artillery fire leaving much of the city in ruins.

A reporter contributing to AFP inside the city on Thursday saw entire floors of buildings that had crashed onto those beneath, while on others, facades were completely blown away to reveal empty, destroyed interiors.

Trenches dug by IS fighters were still visible, as were army minesweepers working to locate and defuse explosives laid by the jihadists.

- Iraqi forces enter Al-Qaim -

Before Syria's war began in March 2011 with anti-government protests, around 300,000 people lived in the city, the capital of Deir Ezzor province along Syria's eastern border with Iraq.

But in 2014, IS jihadists seized the city and much of the surrounding province, including vital oil and gas fields that once served as a key source of revenue for the extremists.

IS has now been driven from most of its strongholds in Deir Ezzor, but it still controls over 35 percent of the province, much of it empty desert.

Its last major position is the town of Albu Kamal, though it also holds a string of smaller towns and villages and at least one oil field, according to the Syrian Observatory for Human Rights monitor.

IS, which at its peak controlled territory roughly the size of Britain, has suffered a string of losses in recent months in both Syria and Iraq.

On October 17, it lost the city of Raqa to the US-backed SDF, a highly symbolic blow that illustrated how its "caliphate" has disintegrated.

In Deir Ezzor province, it is under attack by both regime and SDF forces, while across the border in Iraq it now retains a foothold in just a single town, Al-Qaim, after losing its stronghold of Mosul in July and the town of Hawija in October.

Iraqi forces entered Al-Qaim on Friday, quickly taking several districts, and also recaptured an important border crossing nearby, military commanders said.

Iraq's Joint Operations Command said troops had "regained full control" of the Husaybah border post on the edge of Al-Qaim after launching a push to oust the jihadists.

Government forces launched the operation last week to seize Al-Qaim and its surroundings, a barren pocket of desert along the Euphrates river near the Syrian border.

The US-led coalition has said around 1,500 IS fighters are left in the area, which it expects to be the scene of the "last big fight" against the group in Iraq.

Source: AFP