As many as 22 young British women traveled from the United Kingdom to war-torn Syria in 2014, Deputy Assistant Commissioner Helen Ball, the UK's senior national coordinator of counter-terrorism, told BBC on Sunday.
Ball indicated that four of those 22 girls were aged under 20, stressing that the problem of British girls joining radical Islamists was growing.
According to the US National Counterterrorism Center's estimates, released in February, the militant group, in control of large swathes of Syria and Iraq, has at least 20,000 fighters in its ranks from around the world.
British security authorities estimate that at least 600 people from the United Kingdom have joined the Sunni extremist group.
In the summer of 2014, Turkey banned anyone crossings its border with Syria, except Syrian citizens. However, it was revealed last week that foreigners use fake Syrian documents to pass Turkish border checkpoints.