BEIRUT (REUTERS) - At least 26 people were killed east of the Islamic State in Iraq and Syria-held city of Palmyra on Tuesday, a war monitor said, as Syrian troops backed by Russian air strikes staged attacks near the town a day after Russia started a military withdrawal from Syria.

The Britain-based Syrian Observatory for Human Rights, which monitors the Syrian conflict through a network of sources on the ground, said the casualties occurred during air strikes along the road between the historic city of Palmyra, called Tadmur in Arabic, and a town to its east, al-Sukhna.

Syrian government forces also made major gains to the west of Palmyra, the Lebanon-based Shi'ite group Hezbollah's al-Manar television station reported.

It said the Syrian troops were supported by "heavy air cover from Russian jets".

Last week, the Syrian army backed by Russian air strikes began a campaign aiming to retake Palmyra from ISIS militants to open a road to the eastern province of Deir al-Zor, which is mostly held by the extremists.

The capture of Palmyra and further eastward advances into Deir al-Zor would mark the most significant Syrian government gain against ISIS since the start of the Russian intervention in Syria's civil war in September.

With Russia's help, Damascus has already taken back some ground from ISIS, notably east of Aleppo.

The Observatory said fierce clashes continued between Syrian forces and planes thought to be Russian in areas west of Palmyra.