"We are investigating the signals from this open source to get further confirmation," a spokesperson for the Dutch AIVD intelligence sources told AFP, adding they had no further information.
Syrian activists on Monday said via Twitter that "Daesh executed eight Dutch fighters on Friday in Maadan, Raqa province, after accusing them of attempting desertion and mutiny."
Daesh is another name for IS, which is also known by the acronyms of ISIS or ISIL.
The group said in a statement that the eight were "then buried... In the mountain in Aljerf area, after being accused of incitement against ISIS."
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RBSS has been documenting since April 2014 abuses in Raqa, the group's de-facto capital in northern Syria.
Tension has boiled in Raqa over the past month between 75 Dutch jihadists -- among them fighters of Moroccan origin -- and IS intelligence operatives from Iraq, RBSS said.
According to past estimates by the Dutch secret services, 200 people from the Netherlands including 50 women have joined IS in Syria and Iraq.