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BEIRUT (Reuters) - A war monitor said on Wednesday the Syrian army had seized control of all parts of the Old City of Aleppo which had been held by rebels, part of an advance which has seen insurgents lose around two thirds of their main urban stronghold over the past two weeks.

The Syrian army and allied forces began to enter the Old City on Tuesday, and are looking closer than ever to achieving their most important victory of the five-year-old civil war by driving rebels out of the besieged eastern sector of the city.

The Syrian Observatory for Human Rights said the army advance on Tuesday and overnight, which was supported by heavy air strikes and shelling, caused insurgents to withdraw from the historic Old City, including from the area around the historic Umayyad Mosque.

A Turkey-based official with one of the rebel factions told Reuters government forces had taken part of the Old City, but not all of it.

A military source confirmed to Reuters on Wednesday that the Syrian army had entered Aleppo’s Old City.

Restoring full control over Aleppo, Syria’s most populous city before the war, would be a major prize for Syrian President Bashar al-Assad in the country’s multi-sided conflict.

The war has killed hundreds of thousands of people, made more than half of Syrians homeless and created the world’s worst refugee crisis.