Separate Russian and US-led coalition air strikes on Tuesday killed 35 civilians in Syria's eastern Deir Ezzor province, where parallel offensives against the Islamic State group are under way, a monitor said.
Syrian President Bashar al-Assad has welcomed Russian Defence Minister Sergei Shoigu to capital Damascus as Syrian forces supported by the Russian army prepare to make a final push into parts of the eastern city of Deir Az Zor.
Separately, Syrian state television said on Tuesday Islamic State militants have been using the river to flee the city of Deir al-Zor.
Two sets of strikes 30 minutes apart hit civilians sheltering in tents along the Euphrates and boats on the river, the monitor said.
"Military reinforcements have been arriving since Sunday night to begin the operation to seize control of the city", said Rami Abdel Rahman, who heads the Britain-based Syrian Observatory for Human Rights monitoring group.
The army also captured several key areas in the city and its countryside, such as breaking the siege of the Deir al-Zour airbase, capturing the Teym oil field and the Tharda mountains.
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Much needed aid is finally pouring into the city after a joint Syrian-Russian offensive broke ISIS blockade.
Syrian government troops are fighting to the west of the Euphrates River that slices diagonally across Deir Ezzor province, in an assault backed by Moscow.
Since 2014, IS has held swathes of the province and about 60 percent of its capital, encircling two regime-held enclaves in the western half of Deir Ezzor city.
Separately, the Syrian Democratic Forces, a Kurdish-Arab alliance backed by the US-led coalition, is battling IS east of the river.
It made no mention of regime forces, who are now in control of about half of Deir Ezzor city, and did not say whether the civil council would coordinate with, or rival, government authorities.