Syria's military and allied forces have completely recaptured Deir Az Zor city from ISIL, the armed group's last urban stronghold in the war-torn country.

Friday's announcement by state media came after the army and allied militias eliminated the last Islamic State of Iraq and the Levant fighters from the eastern city that had been under a three-year siege.

"The armed forces, in cooperation with allied forces, liberated the city of Deir Az Zor completely from the clutches of the Daesh terrorist organisation," an unnamed military source was quoted as saying, using an Arab acronym for ISIL.

Deir Az Zor had been almost entirely controlled by ISIL, also known as ISIS, since 2014. It is the main city of the oil-rich province of the same name.

The campaign to retake the city began in September when Syrian forces managed to break the three-year siege of the city by ISIL.

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Al Jazeera's Hashem Ahelbarra, reporting for Gaziantep on the Turkey-Syria border, said Syrian forces will likely find a city destroyed by months of air raids and intense fighting.

"This is a significant development in that it paves the way for the Syrian army to start expanding further in the eastern part of the country," he said.

According to the Syrian Observatory for Human Rights war-monitor group, ISIL still controls about 40 percent of Deir Az Zor province.

Army units - in cooperation with supporting forces - regained control over Deir Az Zor's al-Hamidya neighbourhood, the official SANA news agency said late on Thursday.

The Syrian army and Kurdish-led forces supported by the United States are in a race to retake the rest of the eastern province.

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The forces of Syria's President Bashar al-Assad have been backed by Russian air raids since September 2015, helping to turn the tide in the war against ISIL and other armed groups.

A US-led international coalition, meanwhile, has provided air support to a Kurdish-Arab alliance, the Syrian Democratic Forces (SDF), also fighting ISIL in Deir Az Zor.