With a sudden lunge through extremist lines, the Syrian army and its allies on Monday came to within 3km of relieving the Euphrates city of Deir Ezzor, where the Daesh terror group has besieged 93,000 civilians and an army garrison for years. 

The advance on the eastern city marks another stinging setback for the once-triumphant Daesh, fast retreating in both Iraq and Syria as its self-declared “caliphate” crumbles.

Syrian troops were rapidly approaching the city, reaching a point 3km away, state television said. Dozens of trucks loaded with food stood ready to enter the enclave in the city once government forces break the siege, it said. 

A military media unit run by Hizbollah, a key ally of Damascus, said the advancing forces were heading to the garrison's camp on the city outskirts. 

Deir Ezzor’s provincial governor told Reuters he expected the army could reach the city within hours. 

"Daesh is in confusion. There is no leadership or centralised control," said a commander in the military alliance supporting Syrian President Bashar Assad.

Hemmed in on all sides, Daesh, which ruled over millions of people in both Iraq and Syria at its peak in 2014, is falling back on a last Euphrates stronghold downstream of Deir Ezzor city in the towns of Al Mayadin and Al Bukamal, near the border with Iraq. 

But as it has lost its core territory — defeated in Iraq's Mosul now yielding street after street in Syria's Raqqa — the ultra-hardline group has still been able to launch attacks in the West and maintain a threat in other centres such as Libya.

The fighters have been driven out of nearly all of their territory in Iraq over the past two years by government forces backed by a US-led coalition. In Syria, they are fighting against both Assad’s Russian-backed government and a US-backed Arab and Kurdish militia that has launched an assault on Raqqa.

In the Daesh-encircled pocket in Deir Ezzor, news of the army’s approach prompted people to take to the streets to celebrate, governor Mohammed Ibrahim Samra said by phone.

The city has been cut off since 2013, after rebel groups rose up against Assad during the first flush of Syria’s six-year war. Daesh then overran rebel positions and encircled the army’s enclave in the city in 2014.

It was a major prize. Deir Ezzor is the centre of Syria’s oil industry, a source of wealth to the group and a serious loss to Damascus. As the army has pushed east in recent months, oil and gas fields have once more fallen to the government. 

Daesh fighters stepped up efforts this year to seize the enclave before the army could arrive. In January, they severed it from the city’s military airbase and took over a nearby hill, further straining its links to the outside. 

During the long siege, high-altitude air drops have supplied the city. The United Nations said in August it estimated there were 93,000 civilians in the government’s Deir Ezzor pocket, where conditions were “extremely difficult”. 

“Despite all this and despite the shelling and injured, things are running in the city,” governor Samra had said on Sunday. “The institutions are running the bakeries. Water is also pumped twice a week to our residents, aid is distributed daily.” 

For Assad, the weekend’s lightning advance caps months of steady progress after government forces turned from their victory over rebels in the northern commercial capital Aleppo last December to push eastwards against Daesh.

“The army has been advancing in a rapid and calculated way from all directions,” a Syrian military source said, referring to the months-long campaign across the desert. 

With Russian jets and an alliance of Shiite militias backed by Iran, including Lebanon’s Hizbollah, the army has captured swathes of the central and eastern deserts in parallel offensives from Palmyra and Al Resafa. 

Those offensives have accelerated since linking up last month, taking swathes of land from Daesh except for a small zone near the town of Al Salamiya. The militants still control much of Deir Ezzor province, including half the city. 

Heavy Russian air cover has helped the Syrian military and allied forces march towards the city, Russia’s defence ministry said in a statement on Monday.

A resident of the city reached by telephone, who gave his name only as Mohammad, said he could hear the sound of warplanes in the distance. The army advances over the last two days had sparked “indescribable joy” among people in the enclave after years of siege, he said.