The Syrian army backed by Russian air strikes is aiming to capture the historic city of Palmyra from Islamic State to open a road to the eastern province of Deir ez-Zor in an offensive that got under way this week, a source close to the Syrian government said, according to Reuters.

The Russian air force has hit Palmyra with dozens of air strikes since Wednesday, March 9, according to the Syrian Observatory for Human Rights monitoring group. Syrian government forces were on Friday battling Islamic State some 7 km (4 miles) from the ancient site that fell to the jihadists last May.

Observatory Director Rami Abdulrahman described it as a large-scale assault, calling it a "real operation to retake control". The source close to Damascus said the aim was to "seize the road from Tadmur (Palmyra) to Deir al-Zor."

Islamic State has blown up ancient temples and tombs since capturing Palmyra in what the UN cultural agency UNESCO has called a war crime. The city, located at a crossroads in central Syria, is surrounded mostly by desert.

The capture of Palmrya and further eastward advances into Islamic State-held Deir ez-Zor would mark the most significant Syrian government gain against IS since the start of the Russian intervention. With Russia's help, Damscus has already taken back some ground from IS, notably east of Aleppo.