"At least 36 people were killed and dozens more injured in more than 70 raids carried out by Russian and Syrian planes against several districts in Deir Ezzor," Rami Abdel Rahman of the Syrian Observatory for Human Rights monitoring group told AFP.
He described the raids, which targeted several large cities and smaller towns in the province and three oil fields yesterday, as "the worst bombardment of the region since the start of the uprising in 2011".
The US-led coalition fighting IS said Monday it had destroyed 116 fuel trucks used by the jihadists in eastern Syria, in one of the largest raids in weeks.
IS reportedly makes millions of dollars in revenue from oil fields under its control, and the coalition has regularly targeted oil infrastructure held by the group.
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An investigation by British newspaper The Financial Times last month estimated the jihadists reap some USD 1.5 million a day from oil, based on the price of USD 45 a barrel.
Moscow fired 18 missiles from ships in its Caspian Sea fleet at seven targets in the Raqa, Idlib and Aleppo provinces, defence minister Sergei Shoigu was quoted as saying by Russian news agencies.