British police has launched an appeal to trace three London schoolgirls who are believed to be making their way to Syria to take part in country's conflict, having flown to Turkey earlier this week without telling their families.

The three friends, two aged 15 and one 16, left their east London homes on Tuesday and travelled to Gatwick airport where they caught a Turkish Airlines flight to Istanbul.

Police said on Friday that they were working with Turkish authorities to try to find the girls and bring them home.

"We still think there's a possibility they're in Turkey and that's why we're having the appeal," Richard Walton from London police's counterterrorism command told reporters on Friday.

The three girls, two of whom were named as British nationals Shamima Begum and Kadiza Sultana, were pupils at the Bethnal Green Academy.

They are friends with a fourth teenage girl from the same school whom police believe is already in Syria, having travelled to Turkey in December.

Their families were surprised and devastated by the disappearance of the girls, Walton said.

Security forces estimate some 600 British Muslims have travelled to Syria to join the conflict there, some of them with the Islamic State of Iraq and Levant group.

Around half have since returned, and dozens have been arrested in Britain under anti-terrorism legislation.