Russian airstrikes reportedly cut in half ISIS' proceeds from illegal oil trades

"I think they [ISIS] will attempt to [take over more oil fields], but I do not see ISIS expanding with the Russians there," the source said, according to Fox News.

"The Russians are using their air power far more effectively than we are. Some may say that’s not in accordance with the law of land warfare. But the fact is they are getting results," he added.

Read alsoRussia's costs on Syrian war planned at $3.3 mln a day, grow to $8 mln mid-NovAs UNIAN reported, earlier the Syrian opposition accused Russia of cooperating with the ISIS.

The ISIS makes about $40 million a month on illegal oil sales, raking in close to $500 million a year, U.S. authorities claim. According to them, significant volumes of oil are sold to the government of Syrian President Bashar al-Assad with some finding their way to Turkey.