CCTV footage shows three British teenagers suspected of planning to join Islamic State (Isis) militants in Syria being detained at Istanbul's Sabiha Gokcen airport on 13 March after a tip-off from British authorities that two of them were travelling to Turkey via Spain.

Footage released by authorities at Sabiha Gokcen airport on 17 March purported to show the three male teenagers being taken to the risk and analysis centre after their passports were checked at passport control.

Turkey set up the risk and analysis centre at Istanbul's Sabiha Gokcen and Ataturk airports where intelligence and security officials are working to ward off foreign fighters who come to Turkey to join IS.

The three teenagers, who have not been named, were arrested in London after being deported from Turkey, officials said on 15 March.

London police said they had been made aware on 13 March that two 17-year-old boys from the city had gone missing and were thought to be travelling to Syria. Further inquiries revealed they had travelled with a 19-year-old male, police said.

The arrest comes after three London schoolgirls entered Turkey last month. They are thought to have joined IS in Syria.

Security services estimate some 600 Britons have gone to Syria or Iraq to join militant groups, including Mohammed Emwazi, otherwise known as "Jihadi John", who has appeared in several IS beheading videos.

Hundreds of other Europeans have also joined the fight.

Their involvement has raised fears about the possibility of attacks at home if they return trained and further radicalised.