It's the first claim of responsibility for an attack that claimed 84 lives at a July 14 fireworks display for France's national holiday.
The claim - circulated on social media by a news outlet affiliated with the group - didn't name Mohamed Lahouaiej Bouhlel, the 31-year-old Tunisian who authorities say was behind the wheel as a truck crashed into revelers Thursday night.
But the statement quoting an IS security member said the man was following calls from the group to target citizens of countries fighting it.
What is known publicly about Bouhlel so far suggests a troubled, angry man with little interest in the group's ultra-puritanical brand of Islam. But, in a statement to reporters, Interior Minister Bernard Cazeneuve hinted that Bouhlel may have had a last-minute adoption of a more extremist worldview.
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"It seems he was radicalised very quickly," Cazeneuve said following a ministerial meeting at the Elysee Palace in Paris.
It's also unclear whether or not Bouhlel, who was shot dead by police that night, had been acting alone.
The claim of responsibility came as French security chiefs met in Paris and as Nice's seaside boulevard, the famous Promenade des Anglais, was slowly coming back to life.
A makeshift memorial of bouquets, candles and messages had been set up near one end of the expansive avenue.
The suffering is far from over. Two days after the atrocity, many families are still hunting for missing loved ones, going from hospital to hospital in an effort to find people who've disappeared in the chaos of the truck's rampage Officials said 202 people had been wounded in the attack, with 25 of them on life support as of late yesterday.