Missing schoolgirls: Police believe students have crossed into Syria

It is feared the schoolgirls left the UK last week to join Isis fighters

Kazida Sultana, 16, Amira Abase, 15, and Shamima Begum, 15, (left to right) (Met Office)

Three British school girls feared to have left the UK to join Isis have crossed the Turkish border into Syria, police believe.

Police are attempting to track down Shamima Begum, 15, Kadiza Sultana, 16, and 15-year-old Amira Abase, after they flew to Istanbul from Gatwick Airport last Tuesday.

Scotland Yard said tonight that counter-terrorism officers leading the investigation "now have reason to believe that they are no longer in Turkey and have crossed into Syria".

A spokesman added: "Officers continue to work closely with the Turkish authorities on this investigation."

The announcement comes after Turkey hit back at Britain for suggesting its national airline could have done more to stop the three schoolgirls travelling to Istanbul on their way "to join Isis" in Syria.

Turkish deputy Prime Minister Bulent Arinc accused British officials of taking three days to alert Turkey about the search for the girls, and said the UK would be held accountable if they are not found.

However, Scotland Yard today denied the claims, and said it had been working with Turkish authorities since Wednesday – a day after the girls went missing.

Earlier, Mark Keary, the girls’ head teacher at Bethnal Green Academy in east London said he was "shocked and saddened" by their disappearance, but stressed police had not found evidence that they had been radicalised at school.

He added that access to social media at the school was "strictly regulated", after it emerged that a tweet sent from a Twitter account under Shamima's name was sent to Aqsa Mahmood, who left Glasgow for Syria in 2013 to become a "jihadi bride".

The case underlines fears that growing numbers of young women in Britain and Europe are lured by online propaganda to join the group calling itself the 'Islamic State' and follow in Mahmood's footsteps.

Security officials believe that at least 500 Britons have travelled to Syria to fight with extremists, often via Turkey. Experts estimate about 50 are female.

Relatives of the three schoolgirls have made emotional pleas for them to come home amid fears they may have been recruited by jihadists on the internet.

The latest development from police comes after Prime Minister David Cameron urged airlines and internet firms to do more to prevent British teenagers from travelling to join extremist groups.

The Prime Minister's official spokesman said: "We work very closely with the Turkish authorities on a whole series of security measures. We are going to continue to do so.

Additional reporting by PA