The report by the Britain-based Syrian Observatory for Human Rights, a pro-opposition watchdog with a network of activists in Syria, could not be independently verified, but a pro-opposition media group based in the province where the killings allegedly took place confirmed the information.
The punishments were carried out against the women as well as their husbands in public on Sunday and Monday in Dair Alzour province, according to the observatory. Much of the province is under Islamic State control.
Islamic State, which espouses a harsh interpretation of Sharia, or Islamic law, has carried out more than 3,000 executions in Syrian territories in the last year, the observatory said. Its execution methods have included beheading, stoning, crucifixion and throwing people from buildings.
The reported beheadings came days after Islamic State fighters were reported to have carried out hundreds of house-to-house killings, some of them of women and children, in the Syrian town of Kobani, on the Turkish border.
Kurdish fighters, backed by U.S. airstrikes, had ejected Islamic State from the town in January after a months-long siege. Activists said Islamic State's incursion appeared aimed at exacting vengeance rather than trying to retake Kobani.
The extremist group earlier in June suffered a substantial battlefield setback when Kurdish-led fighters seized another Syrian town on the Turkish border, Tal Abyad, cutting a major supply route to Islamic State's de facto capital, Raqqa.
The Sunni militant group has carried out many highly publicized beheadings of male captives, including American journalists James Foley and Steven Sotloff, as well as aid workers and Coptic Christians. And women have been publicly executed as well in Islamic State-controlled areas, with some of them stoned to death.
But Islamic State searches tirelessly for ever more baroque punishments to up the shock value, many of them meticulously chronicled in grisly videos.
Last week, it released video depicting the executions of accused "spies" by drowning some in a cage lowered into a pool, and blowing others up with explosives looped around their necks. In February, they locked a captured Jordanian pilot in a cage and burned him to death.
The group had boasted that a campaign of attacks would take place during the Muslim holy month of Ramadan, which began two weeks ago.
In the last week, affiliates of the group have claimed responsibility for a shooting assault on a beach resort in Tunisia that killed 38 people, mostly European tourists; the suicide bombing of a Shiite Muslim mosque in Kuwait, which killed 27 worshipers, and a car bombing outside a funeral observance in Yemen late Monday that killed nearly 30 people.
Staff writer King reported from Cairo and special correspondent Bulos from Molyvos, Greece.