Islamic State militants captured a string of villages near Aleppo on Friday in a lightning attack that brought them closer to the prized city in northern Syria, despite a major increase in Russian airstrikes that Moscow insists are targeting the extremists.

The surprise advance that brought the IS group to within a few miles of Aleppo was the most significant in months, following what has largely been a summer of stalemate in the war against the extremists.

It comes amid a wave of intense Russian airstrikes that have targeted insurgents fighting to topple President Bashar Assad, and a ground offensive by the Syrian army in the country's central region.