VICE News The terror group ISIS made gains near Syria's largest city in the last 24 hours, indirectly aided by Russian airstrikes that drove other groups out.

ISIS militants seized five villages on the northern edge of Aleppo, putting them within 1 mile of territory held by the Syrian regime, Reuters reports.

Russian airstrikes in the area reportedly killed hundreds of anti-regime rebels, many of whom ISIS (also known as the Islamic State, ISIL, and Daesh) was also fighting.

"Daesh has exploited the Russian air strikes and the preoccupation of the (rebel) Free Syrian Army in its battles in Hama, and advanced in Aleppo," one rebel commander told Reuters.

Hassan Hassan, an expert on Syria who recently coauthored the book "ISIS: Inside the Army of Terror," told Business Insider that ISIS' gains near Aleppo are a "big win."

"I’m sure ISIS has sleeper cells in Aleppo," Hassan said. "If they pushed, if they held their ground then that would be … a big win for them.

"They could actually establish a foothold inside Aleppo, and it also helps them to make more progress in Hama," another key city in western Syria.

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So far, Russia's campaign against rebels in Syria has hit anti-regime fighters about 90% of the time while leaving ISIS strongholds untouched. And up until now, rebel groups have been successful keeping ISIS out of Aleppo.

There are many factions fighting for control of Syria — ISIS sometimes fights forces loyal Assad and seeks to gain as much territory as it can in Syria, but the group mostly fights nationalist rebels as well as other jihadist groups like the Nusra Front, al-Qaeda's affiliate in the country.

Russia went into Syria claiming that its primary target was ISIS, but it seems that its real goal is to prop up Assad, who is a close ally of Russian President Vladimir Putin.

And as Russian airstrikes weaken the nationalist opposition and ISIS' jihadist rivals, ISIS is taking advantage of opportunities to move into territory it wasn't previously able to secure.

Gains in Aleppo "would basically give them the ability to attack the rebels from different sides and take areas under the control of the rebels," Hassan said.

"They will try this because they see the rebels at their weakest points and they want to fortify the front from different sides," he said.

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Furthermore, Russia has something to gain from leaving ISIS largely untouched.

"The Russians are following the same script as the Syrian regime, which is basically to avoid ISIS as much as they can because ISIS is useful," Hassan said.

"ISIS can attack the rebels. It’s not in the heartlands, it's in areas that are farther away. ... If ISIS attacks the rebels in Aleppo, that’s good news for the Russians and the regime."

Consequently, Russia would hit ISIS harder only if the group creeps into regime strongholds.

"The regime and Russians will focus on the heartlands — Hama, Damascus, but not Palmyra," which is an ISIS stronghold, Hassan said.

ISIS also reportedly killed a top Iranian commander in northern Aleppo this week, despite Russian airstrikes. Iran is allied with Russia and Syria while commanding proxies in the country.