Contrary to what Moscow would like the world to believe, Russian airstrikes in Syria are not only not hurting the Islamic State (ISIS) but are actually helping the jihadist group, a leading British defence analyst disclosed.

Speaking to Express.co.uk, Justin Bronk, an expert in foreign conflicts, said Russian President Vladimir Putin's bombing campaign is actually "helping ISIS because they are attacking in very concentrated fashion moderate rebel groups that the US backs."

Bronk, the research analyst at the Royal United Service Institute, a think tank, said only 5 percent of Russia's more than 600 bombing sorties had hit ISIS positions.

He said the Russians are trying to depict the war as a choice between Syrian President Bashar al-Assad or ISIS rather than a choice between Assad and the moderate Syrian opposition that excludes ISIS, which is how the US prefers the world to see the conflict.

The Russians have been "quite skilled at that. They haven't really attacked ISIS, maybe 5 percent of their strikes. There is not much reason why ISIS would have a grudge against Russia," Bronk said.

He made his comments as ISIS militants, apparently unaffected and undaunted by the Russian airstrikes, seized another Syrian town, Maheen in the southwest of Homs province, from government forces on Sunday, the Britain-based Syrian Observatory for Human Rights (SOHR) reported.

ISIS forces were also reported to be attacking Sadad, a neighboring Christian town. SOHR said 50 Syrian government soldiers were killed or wounded during the ISIS attack on Maheen.

The ISIS advance brings the jihadist group closer to a highway linking Damascus to the central Homs province, threatening to cut off government supply routes.

While ISIS militants were attacking Maheen and Sadad towns, Russian warplanes reportedly bombed the outskirts of ISIS-held Palmyra, a UN heritage site where ancient structures still stand.

Moscow claimed that the latest Russian airstrikes destroyed more ISIS targets. However, sources said Russia is concentrating its firepower on positions held by anti-Assad rebel groups.

Human rights groups have reported an upsurge in civilian deaths in rebel strongholds that are being pounded by Russian warplanes.