BEIRUT, Lebanon — The Islamic State registered significant gains on Friday in the area of northwestern Syria that Russian warplanes have been bombing, taking six villages near Aleppo and threatening to cut off an important route north to the Turkish border. Late in the day, there were reports that rebels had reasserted control in one village.

The Kremlin has said its military had entered Syria to fight the Islamic State, but the Russian forces have concentrated much of their firepower on insurgent groups aligned against President Bashar al-Assad, including the Nusra Front, Al Qaeda’s affiliate in Syria, and relatively secular groups like the Free Syrian Army. Rival insurgents say that the Islamic State, also called ISIS or Daesh, is taking advantage.

“Daesh has exploited the Russian airstrikes and the preoccupation of the Free Syrian Army in its battles in Hama, and advanced in Aleppo,” one rebel commander told Reuters.