Syrian government troops and allied forces have broken a three-year siege imposed by fighters from the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) on a government enclave in the eastern city of Deir Az Zor, according to Syria's state media.

Backed by Russian air support, the army and their allies had been advancing towards Deir Az Zor on several fronts in recent weeks, and on Tuesday reached the western outskirts of the city after ISIL defences collapsed.

State news agency SANA said on Tuesday that the advancing forces had joined up with fellow soldiers at the previously besieged government-held airbase known as Brigade 137 on the outskirts of Deir Az Zor city.

Ali Mayhub, a brigadier general in the Syrian armed forces, said in a statement that "after a series of successful operations, our heroic forces and our allies with the backing of the Russian forces managed to lift the siege on our people in Deir Az Zor city".

He added: "Our forces managed to break the ISIL defence lines and meet with our fellow comrades who held the ground in Deir Az Zor and protected its people, a great example of sacrifice and glory”.

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ISIL controls most of Deir Az Zor province - apart from a Syrian government-held enclave in Deir Az Zor city and a nearby military airbase.

Al Jazeera's Mohammed Jamjoom, reporting from Beirut in neighbouring Lebanon, said: "The Syrian army and fighters from the Lebanese armed group, Hezbollah, are trying to get deeper inside the city and are also pushing towards the Deir Az Zor airport."

Syrian President Bashar al-Assad congratulated the government troops on Tuesday's advance, his office said.

"Today you stood side-by-side with your comrades who came to your rescue and fought the hardest battles to break the siege on the city," Assad said in a call with the commanders of troops who had been besieged in a base in Deir Az Zor.

State media reported celebrations inside the government-held parts of the city as troops arrived.

The Syrian Observatory for Human Rights, a British-based war monitor, said a nearby military airbase in the south of the city and three adjacent neighbourhoods were still under siege by the armed group.

Around 100,000 people are believed to be inside government-held areas of Deir Az Zor, with perhaps 10,000 more in parts of the city held by ISIL.

"The coming days will see the clearing of the city of Deir Az Zor (from ISIL fighters) and the start of advances on nearby countryside held by Islamic State," Mohammed Ibrahim Samra, Deir Az Zor provincial governor, told Reuters news agency.

'ISIL on the back foot'

Syria's army began its offensive to reach Deir Az Zor last month and has advanced on multiple fronts, including from the neighbouring Raqqa province to the west and central Homs province to the south.

The latest developments mark a strategic and symbolic defeat for ISIL, which last month lost its hold over Iraq's second largest city of Mosul. It has also lost more than half of its Syrian stronghold to US-backed Syrian forces in its self-proclaimed capital of Raqqa.

With the latest breach in Deir Az Zor, Al Jazeera's Jamjoom said that ISIL appears to be "on the back foot".

"The allied forces are going full steam right now trying to retake as much territory as possible," he said.

"It is going to be a difficult fight. But by all accounts, including that of Syrian state TV, it is believed that the government forces will be able to fully take control over Deir Az Zor in the weeks ahead."