Stockholm, Aug 10 (IANS): A pregnant 15-year-old Swedish girl and her 19-year-old partner have been captured by Islamic State (IS) militants in Syria, the media reported on Monday.

According to local Swedish media reports, the girl telephoned her parents and said that she and the older boy were being held by the IS, The Local news portal reported.

"It really is a critical situation," her father told regional newspaper Boras Tidning.

He said that the couple had married in a Muslim ceremony earlier this year and suggested that the pair had been questioned about their marriage by the extremist group because they were unable to provide any formal documentation to prove their partnership.

He said that his daughter told him that an IS court was now considering the case and that the pair was asked to "swear allegiance" to the radical group in the meantime.

According to Swedish laws, both partners have to be 18 years and above to be legally married.

No details have emerged about how the couple were captured, but according to the girl's father, she has twice been able to contact her family in southern Sweden after being secretly lent a phone by a woman in Syria.

Sweden's Security Service, Sapo, said that around 150 Swedes have fled to Iraq and Syria to fight alongside the extremists.