THREE schoolgirls thought to have been lured into fighting for the Islamic State by Scots jihadi bride, Aqsa Mahmood, have crossed the border into Syria, according to police.

Shamima Begum, 15, Kadiza Sultana, 16, and 15-year-old Amira Abase sparked a police hunt after they flew to Istanbul from Gatwick Airport last Tuesday, days after contacting Miss Mahmood on Twitter.

Scotland Yard said 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."

Earlier police denied claims that they failed to contact Turkish authorities about the three girls.

Turkish deputy prime minister Bulent Arinc told reporters that British officials would be accountable should the search prove fruitless because of a three-day delay in alerting the country.

But Scotland Yard said they began working with Turkish authorities the day after the first two teenagers were reported missing a week ago.

It came as Miss Mahmood's Twitter account was finally suspended days after claims she had "groomed" the runaway schoolgirls via the social media site.

Miss Begum was following more than 70 known extremists on Twitter and sent an open message to Miss Mahmood, a 20-year-old student from Glasgow, asking her to start a private communication four days before she and her friends fled the country.

It is unclear whether Miss Mahmood, who has been in Syria since November 2013, responded.

Meanwhile, David Cameron has urged airlines and internet companies to do more to prevent radicalised British youngsters from travelling to the Middle East.

He said internet companies must live up to their "social responsibility" by taking down extremist content and improving cooperation with the authorities over contacts between extremists and young people vulnerable to radicalisation.

Turkish Airlines has also come under fire for allowing the teenagers to board without questioning why they were travelling without an adult.

The Prime Minister said that border police should be alerted of any concerns so they can stop individuals from travelling.

Mr Cameron said "proportionate arrangements" with airlines were needed to ensure potentially at risk children are identified and questioned at check-in.

A public appeal for information about the missing girls was launched by police on Friday, three days after the girls boarded their flight to Turkey.

Mr Arinc criticised the police for not taking "necessary measures".

He said: "It is a condemnable act for Britain to let three girls ... come to Istanbul and then let us know three days later.

"They haven't taken the necessary measures.

"The search is ongoing. It would be great if we can find them. But if we can't, it is not us who will be responsible, but the British."

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.

"It is a good, strong, constructive relationship. We are working with them on this case.

"We are going to keep working very closely with the Turkish authorities on what is a shared challenge."

Relatives of the three schoolgirls have made emotional pleas for them to come home.

Earlier, the girls' headteacher said he was "shocked and saddened" by their disappearance, but said police had not found evidence that they were radicalised at school.

Mark Keary, principal of Bethnal Green Academy in east London, said police spoke to the girls after another student disappeared in December and indicated at the time that there was no evidence that they were at risk of being radicalised or absconding.

He also said access to social media at the school was "strictly regulated".