Relatives have not heard from any of the girls for more than a year when one of the teenagers Kadiza Sultana died in an air strike in the Syrian city of Raqqa, the de-facto capital of the Islamic State (ISIS) in the Middle East.
The families of the three other girls had previously been in intermittent contact via telephone and the Internet but have not heard from them since May 2016, 'The Sunday Times' reported.
A relative of Sharmeena Begum, the first of the four school friends to travel to Syria, said "Only Allah knows. It's heartbreaking for the family".
The schoolgirls, who were high-scoring pupils at an academy in Bethnal Green, east London, made headlines when they became the youngest British recruits to join the ISIS, aged only 15 and 16, from December 2014 to February 2015.
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At least three of the girls, Shamima Begum, Amira Abase and Sultana, married terrorist fighters after leaving the UK together.
Amira's father Abase Hussen last week pleaded to know his daughter's fate.
"You need to find out whether she is [alive] or not," he said.
Theschoolgirlsare among more than 800 Britons who are believed to have left the UK to jointhe ISISor other militant groups in Syria and Iraq.