BEIRUT (Reuters) - Syria’s army and its allies raced towards their enclave in the Euphrates city of Deir al-Zor after a rapid advance on Sunday, seeking to relieve it after years of Islamic State siege as jihadist defenses suddenly collapsed.

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Deir al-Zor’s provincial governor, Mohammed Ibrahim Samra, told Reuters in a phone interview that he expected the army to arrive at the city within hours and that some people there had taken to the streets to demonstrate their happiness.

Syrian President Bashar al-Assad has focused this year on the campaign in Syria’s desert, striking eastwards in a multi-pronged assault to restore Deir al-Zor, where Islamic State has held half the city and all the land around since 2014.

Sunday’s advance brought the army and its allies to about 10 km (6 miles) from the city, said the military media unit run by the army’s ally, Lebanon’s Hezbollah group. On Friday, it reported the army was still 30 km (19 miles) away after capturing Jebel al-Bashri.

A war monitor, the Britain-based Syrian Observatory for Human Rights, said the army had advanced even farther, reaching the border of the besieged garrison’s army camp on the western edge of Deir al-Zor.

Intense fighting is taking place as the army attempts to break through Islamic State lines to join up with the besieged garrison, the Observatory reported.

It is a sign of the rapidly changing fortunes in Syria’s civil war, as Assad, aided by Russian air strikes and Iran-backed Shi’ite militias including Hezbollah, has won a series of battlefield victories.

Islamic State, which took advantage of chaos caused by the civil war to seize swathes of Iraq and Syria in 2014, is retreating in both countries and falling back on its Euphrates valley strongholds on either side of Deir al-Zor.

The United Nations says there are about 93,000 people in the besieged, government-controlled enclave, which includes much of the city, the Brigade 137 military camp and the airport.

For years, Deir al-Zor has been supplied only by air, and the United Nations said last month it had carried out about 300 high-altitude air drops of food over the past year alone.

Syrian state television reported that Islamic State was in a state of collapse in the face of the army’s progress. A Syrian military source said on Saturday the army had also advanced eastwards from Sukhna, on the main road between Damascus and Deir al-Zor, along a parallel route of attack.

Pro-government forces are also fighting to eradicate a large Islamic State enclave they left to their rear in central Syria as they advanced towards Deir al-Zor.