The teenager, who has been put on an international watchlist, is said to have called home a few days ago, telling her mother that she had made it to Syria after travelling through Turkey with her boyfriend.
According to Sweden’s Aftonbladet newspaper, which has spoken to the girl’s mother, the line was crackly, but the teenager suggested that she was planning to join al-Nusra, an extremist Islamist group with links to al-Qaeda.
The girl told her family that she was currently in Homs but that the couple were planning to travel to another city shortly, adding that she would live with a group of women while her partner began military training.
“I am waiting to hear back from them,” the teenager’s mother is quoted as saying by Aftonbladet.
“I cannot describe how terrible this is and I just wish I could go there myself and fetch her.”
The schoolgirl, who has not been named by Swedish media, has been missing from her home in southern Sweden since May 31st.
Her disappearance is being investigated by regional police in Halland in southern Sweden, the operational department of Sweden's national force (NOA, Nationella operativa avdelningen) told The Local on Monday.
“The girl is internationally wanted as missing by Interpol,” added press spokeswoman Carolina Ekéus.
She was however unable to confirm reports that the girl had made it to Syria.
News of the missing girl’s apparent journey comes as concerns grow about the number of young girls leaving one of the safest countries in the world to join radical groups in the Middle East, with reports that at least 30 have travelled from Sweden to Iraq or Syria over the past year.
Last month Magnus Ranstorp from the Swedish Defence University told The Local that some girls “look up to the fighters like pop stars.”
“For a lot of the girls this is a form of emancipation in a perverse way. They think they are getting away from the leash of patriarchal structures in their families or the areas where they live,” he said.
Last week, Sweden's government announced it was considering drafting new legislation that would ban its nationals from fighting in armed conflicts for terrorist organisations such as Isis (also known as IS or the Islamic State).
“It is completely unacceptable that Swedish citizens are travelling to [join] IS, financing the organization, or fighting for it,” Justice Minister Morgan Johansson and Home Affairs Minister Anders Ygeman wrote in a joint article in Sweden's Dagens Nyheter newspaper.
“We have a responsibility for what our citizens do both here, at home and in other countries,” they said.