Forces loyal to Syria’s President Bashar al-Assad flash victory signs and carry a Syrian national flag on the edge of the historic city of Palmyra in Homs Governorate, in this handout picture provided by SANA on March 26, 2016.

Syrian government forces advanced into the ancient city of Palmyra on several fronts on Saturday, backed by waves of artillery barrages and Russian air strikes, a monitoring group said.

The Syrian Observatory for Human Rights said the fighting was the heaviest yet in the army’s three-week campaign to recapture the desert city from ISIS fighters.

Russian warplanes deployed in Syria have made 40 sorties around the city of Palmyra over the past 24 hours, hitting 158 ISIS targets and killing more than 100 militants, Russian news agencies quoted Russia’s defense ministry as saying on Saturday.

It said Syrian soldiers and allied militias had taken control of neighborhoods in the western and northern parts of the city.

Syrian state media earlier said the army had taken full control of Al-Amiriya district on the northern edge of Palmyra.

Two Lebanese TV stations showed footage Saturday of Syrian tanks and machine guns firing at positions inside the town, with smoke rising over the skyline.

The Observatory says ISIS killed at least 10 soldiers in a counter-offensive earlier in the day.

Many of the Palmyra’s Roman-era relics, including the 2,000-year-old Temple of Bel and the iconic Arch of Triumph, have been blown up by ISIS. The town, a UNESCO world heritage site, has been held by ISIS since May.

Palmyra had a population of 50,000 according to a census more than 10 years ago. Those numbers were swelled hugely by an influx of people displaced by Syria’s conflict, which has raged since 2011, but most fled when ISIS took over.

Recapturing the city would open up eastern Syria, where ISIS controls most of the Euphrates Valley provinces of Deir al-Zor and Raqqa, to the army.

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