Syrian army reinforcements arrived in Deir Az Zor on Monday for a new push against ISIL as a second day of air strikes reportedly killed 19 civilians in the area.
Since 2014, ISIL has controlled most of Deir Az Zor city and the surrounding province, which borders territory the armed group holds in Iraq.
Government troops have breached ISIL's siege, captured the strategic Jabal Thardah region, and expanded control to half of Deir Az Zor city.
"Huge military reinforcements - including equipment, vehicles and fighters - have arrived in Deir Az Zor ahead of an attack to push Daesh from the city's eastern neighbourhoods," said Syrian Observatory for Human Rights chief Rami Abdel Rahman, using an Arabic acronym for ISIL.
"Russian and Syrian regime warplanes are striking IS positions in the city and its outskirts."
Deir Az Zor city is the capital of the oil-rich eastern province of the same name, regarded as a strategic prize by both Russian-backed Syrian troops and US-backed fighters.   
Syrian forces have scored major advances in recent days, breaking a pair of ISIL sieges on the city and capturing territory around it.
However, the rapid advance has been accompanied by deadly air strikes with the Observatory reporting 19 civilians were killed on Monday in suspected Russian air raids northwest of the city.
The strikes came a day after the Britain-based war monitor said Russian strikes killed 34 civilians southeast of the city as they fled across the Euphrates River.
Two sets of air strikes 30 minutes apart hit civilians sheltering in tents along the Euphrates and boats on the river, the monitor said.
The attacks come as the Syrian army prepares to push into the eastern ISIL-held part of Deir Az Zor city, according to the Observatory.