The Syrian army and its allies have taken full control of Deir al-Zor city from Daesh, the largest and most important city in eastern Syria, the Syrian Observatory for Human Rights said on Thursday.
There was no immediate comment from Syrian state media, but a commander in a military alliance fighting in support of Damascus said that while advances had been made, some Daesh-held pockets still remained in the city.
The Syrian army launched its attack against Daesh there in September, backed by Russian air strikes and fighting alongside Iran-backed militias and the Lebanese Shi'ite group Hezbollah.
The Islamist group had held most of the city since 2014, except for one large pocket where Syrian army troops and 93,000 civilians had been holed up for three years.
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The provincial capital is located on the western side of the Euphrates River. The Syrian Democratic Forces (SDF), backed by the US-led coalition, launched their own attack on Daesh in the province in September, advancing along the eastern side of the river.
The city's strategic significance for Daesh stemmed from its location near the Iraqi border, where the group also controlled territory, as well as its importance as the oil-producing province's capital.
The recovery of the eastern province's capital underscores President Bashar al Assad's re-established control over the eastern part of Syria.
As the army has pushed east in recent months, oil and gas fields have once more fallen to the government, as well as to the SDF.
Daesh still controls parts of Deir al-Zor province, its territory extending across the border to neighboring Iraq where Iraqi forces are waging a separate offensive against the militant group.
The Islamist group also controls a small pocket in Hama province in western Syria, and a besieged enclave south of Damascus where it is fighting rebel factions.
The Syrian army is also advancing on Albu Kamal city in Deir al-Zor province, located on the Iraqi-Syrian border. Russian bombers struck Daesh targets in the border town on Thursday, RIA news agency reported.