BEIRUT (Reuters) - Russian-backed Syrian government forces and their allies fought their way into Palmyra on Wednesday, driving back Islamic State militants who have held the historic city since December, the Syrian Observatory for Human Rights monitoring group reported.

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A Hezbollah-run media outlet earlier reported that the Syrian army and its allies had recaptured the Palmyra citadel, on the city’s western outskirts, and seized a modern palatial complex to the southwest.

Islamic State has captured Palmyra, whose ancient ruins are a UNESCO World Heritage Site, twice during Syria’s six-year conflict.

The army recaptured the city from the ultra-hardline group in March last year, but Islamic State seized it again in December.

The group has razed ancient monuments during both of its spells in control of Palmyra - destruction the United Nations has condemned as a war crime.

A Syrian military source told Reuters earlier on Wednesday: “The army’s entry to the city will begin very soon.”

The army said it had captured an area known as the “Palmyra triangle” a few kilometers (miles) west of the city after rapid advances in recent days backed by Russian air strikes.

The Syrian Observatory for Human Rights, a British-based organization that reports on the war, said government forces were expected to storm Palmyra at “any moment”. Russia has said its aircraft are supporting the army offensive in Palmyra.

Photos published on an Islamic State Telegram account on Wednesday showed the group’s fighters firing at the Syrian army with rockets and a tank. Reuters could not verify the authenticity of the photos.

Islamic State first captured Palmyra from the government in 2015. During its first period in control of the site, the jihadists destroyed monuments including a 1,800-year-old monumental arch.

Most recently, Islamic State has razed the landmark Tetrapylon - a platform with four columns at each corner - and the facade of Palmyra’s Roman Theatre. Palmyra, known in Arabic as Tadmur, stood at the crossroads of the ancient world.

The government and its allies lost Palmyra as they focused on defeating Syrian rebel groups in eastern Aleppo. The rebel groups were driven from eastern Aleppo in December, the government’s biggest victory of the war.