Syrian government forces are pictured standing beside a sign saying "Welcome to Deir al-Zour" (Photo: AFP)

According to a Syrian military source on Monday, commanders are retreating, leaving behind the local militants, amid a state of collapse in the ranks of IS fighters as a result of the wide-scale offensive by the Syrian army and allied troops.

Syrian forces on Monday were advancing from two directions to break the siege on the Brigade 137 base in the western part of Deir al-Zour. The Brigade now contains 5,000 Syrian soldiers. The provincial authority of Deir al-Zour said the Syrian forces are expected to reach the base by Tuesday evening to break the siege on 93,000 civilians in the city.

Earlier in the day, two Russian soldiers were killed by IS shelling near Deir al-Zour. The Russian air force is heavily striking IS positions there to pave the way for ground forces to enter the city.