BEIRUT (Reuters) - 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 al-Zor in an offensive that got under way this week, a source close to the Syrian government said.

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The Russian air force has hit Palmyra with dozens of air strikes since Wednesday, 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 U.N. cultural agency UNESCO has called a war crime. The city, located at a crossroads in central Syria, is surrounded mostly by desert.

The Islamic State group is not included in a cessation of hostilities agreement that took effect on Feb. 27 and has brought about a lull in fighting between the government and rebels battling President Bashar al-Assad in western Syria.

Since the Russian air force intervened in support of Assad last September, tilting the military balance his way, Western states have criticized Moscow for directing most of its air strikes at rebels in western Syria rather than IS.

The capture of Palmrya and further eastward advances into Islamic State-held Deir al-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.

ISLAMIC STATE LOSING MOMENTUM

The momentum has turned against Islamic State since its rapid advances two years ago following the capture of the Iraqi city of Mosul. Its finances are also under strain, with fighters’ pay cut by up to a half.

The group’s tactics in Syria appear to reflect the strains, as it turns to suicide missions seemingly aimed at causing maximum casualties rather than sustainable territorial gains.

Russian air power has helped government troops backed on the ground by Lebanon’s Hezbollah and the Iranian Revolutionary Guards to advance in strategically vital areas of western Syria where fighting has largely subsided due to the truce.

The source close to the Syrian government said the bulk of the forces mobilized for the Palmyra offensive were from the Syrian army. The source, who is non-Syrian but familiar with military events in Syria, was speaking anonymously because of the sensitivity of the matter.

Abdulrahman of the Observatory said at least 32 Islamic State fighters were killed on Thursday in the Palmyra area. Syrian military officials could not be reached for comment.

Islamic State is being fought in Syria in two separate campaigns: on the one hand by the Syrian government and its allies, and on the other by a U.S.-led alliance that is working with Syrian groups including the Kurdish YPG militia.

The group captured nearly all of Deir al-Zor province after seizing the Iraqi city of Mosul in 2014. The Syrian government, however, still controls part of the city of Deir al-Zor, which is besieged by Islamic State fighters, and a nearby air base.