A fighter of the Islamic State (ISIS)
A fighter of the Islamic State (ISIS) Reuters

CCTV footage has emerged apparently showing three British schoolgirls who left the UK to join ISIS in Syria.

Close friends Kadiza Sultana, 16, and 15-year-olds Shamima Begum and Amira Abase, boarded a flight from London Gatwick to Istanbul in February, and have not been heard from since. Their families launched a public appeal for them to return home.

They are believed to have joined a growing number of Muslim girls and women from western countries leaving for Syria to become "jihadi brides" in the Islamic State terror group's so-called "Caliphate".

The footage was taken on February 17-18 and at Bayrampasa bus station in Istanbul shortly after the girls arrived in Turkey, according to the Milliyet newspaper.

Turkey is a regular route for would-be jihadists to enter Syria, and Ankara has been criticized for not doing more to stem the flow. For its part, however, the Turkish government has blamed British authorities for not tipping them off about the girls, noting that at least one of them was known to have contacted another British female recruit currently in Syria via Twitter.

The video emerges just days after it was revealed that another British Muslim, Mohammed Emwazi, is real identity of ISIS's notorious masked executioner known as "Jihadi John."

British intelligence say at least 500 UK citizens are known to have joined jihadists in Syria and Iraq, although some estimates are much higher.