The Britain-based Syrian Observatory for Human Rights said a woman and child were among the dead in the strikes on Al-Bab, which has been held by IS since early 2014.
Elsewhere in the province, the monitor said at least three people, including a child, had been killed in rebel fire on the government-held area of Aleppo city.
Syria's state news agency SANA put the toll at four dead.
Aleppo city was once Syria's economic hub, but it has been ravaged by war and divided between government control in the west and rebel control in the east since shortly after fighting there began in mid-2012.
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South of the city, meanwhile, government troops backed by Russian air strikes seized several villages from rebel forces, SANA said.
The state news agency said the army had taken three villages south of Aleppo, with the Observatory confirming the advance.
Syrian government troops have launched several ground offensives against rebel forces since Russia began a bombing campaign on September 30.
So far, the offensives have registered only modest advances, though the Russian strikes have reportedly boosted morale among government troops.