The Swedish foreign ministry remained tight-lipped about the case, confirming only minimal details.
"We have been informed that a Swedish minor is in Syria. We are in contact with family members," ministry spokesman Gabriel Wernstedt told AFP, refusing to disclose any other details.
Swedish daily Expressen and local paper Boras Tidning reported that the 15-year-old girl, whose name was not disclosed, disappeared from her foster home in Boras, near the southwestern town of Gothenburg, on May 31.
She and her 19-year-old boyfriend reportedly travelled to Syria via Turkey, and were recruited on arrival by an Al-Qaeda-linked group.
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The couple were captured by IS fighters in the northern city of Aleppo in early August, and have been moved to an IS-controlled area, media reports said, adding that the boyfriend was now being forced to fight for the jihadist group.
The couple were married in a Muslim ceremony in Stockholm earlier this year, without their parents' knowledge.
The daughter has called her parents at least three times while being held by the IS group, after women secretly loaned her a cell phone, the reports said.