A 17-year-old Bosnia-born Austrian girl who left home in 2014 "to serve Allah" by joining the Islamic State (ISIS) in Syria has reportedly been killed after she tried to escape Raqqa, the de-facto ISIS capital and stronghold.

Samra Kesinovic became a "poster girl'' for ISIS after she fled to Syria in April 2014 together with her 15-year-old friend, Sabina Selimovic. The pair appeared on social networking sites brandishing Kalashnikov rifles until recent reports said she was beaten to death by ISIS militants, according to the Daily Mail.

Selimovic was also believed to have died in fighting in Syria, the report said.

"We received information just recently about two 15-year-old girls, of Bosnian origin, who left Austria, where they had been living in recent years; and everyone, the families and the intelligence services of the two countries, is looking for them,'' said David Scharia, a senior Israeli expert of the United Nations Security Council's Counter-Terrorism Committee (CTED).

"Both were recruited by Islamic State. One was killed in the fighting in Syria, the other had disappeared,'' he added.

The pair, whose parents are Bosnian refugees, initially disappeared from their respective homes in Vienna but left a note telling their parents: "Don't look for us. We will serve Allah—and we will die for him,'' the Mirror reported.

The Austrian tabloid Österreich quoted an insider who said Samra was caught as she tried to escape from a house she was sharing with other women in Raqqa.

Citing Austrian papers, the Mail said an Islamic preacher from Bosnia living in Vienna known as Mirsad O., and known by the Islamic name of "Ebu Tejma," had brainwashed the girls to join and fight with the ISIS.

Mirsad O. denied the accusations but police arrested him in November for his role in the alleged terrorist funding network based in Austria.

In an interview with French weekly Paris Match, Sabina said after arriving to Turkey from Austria that they crossed over the border into Syria on foot then ended up in the city of Raqqa "with nothing other than the clothes they were wearing.''

Sabina said she was allowed to speak to the magazine by her husband, a soldier, who was with her in the room when she wrote her answers.

"Here I can really be free. I can practice my religion. I couldn't do that in Vienna,'' she said.

Austrian authorities have expressed concern at the rising number of teenagers leaving the country to fight for ISIS. Austrian Interior Ministry spokesman Alexander Marakovits said youngsters suspected of having ran away from their homes should be swiftly caught before they leave the country.

"Once they have left the country, even if they then changed their minds, it is then almost impossible to get them back,'' Marakovits said.

As many as 130 people from Austria are now believed to be fighting as jihadists abroad. At least half of them originally come from the Caucasus region of Russia and were granted asylum in Austria after the bloody Chechen war.