Media playback is unsupported on your device Media caption BBC security correspondent Gordon Corera assesses whether warning signs were missed

Three missing teenagers believed to be heading to join Islamic State militants are thought to have crossed into Syria, UK police say.

The London schoolgirls were smuggled into Syria from Turkey four or five days ago, the BBC understands.

Sources suggest the girls entered Syria near the Kilis border crossing.

Bethnal Green Academy pupils Shamima Begum and Amira Abase, both 15, and Kadiza Sultana, 16, travelled to Istanbul on 17 February.

They flew from Gatwick to Turkey after telling their parents they were going out for the day.

The Metropolitan Police said it "now had reason to believe that they are no longer in Turkey and have crossed into Syria".

"Officers continue to work closely with the Turkish authorities on this investigation," the force said in a statement.

'Out of reach'

Sources inside Syria suggest three girls who were identified as British - including one said to be wearing glasses - crossed the border with the help of people smugglers.

The news comes after the girls' families made a number of emotional appeals for them to return.

BBC correspondent James Reynolds said it was "not clear what - if anything - British or Turkish authorities are now able to do".

He said the fear was that once inside Syria, the missing schoolgirls would be "out of reach".

Image copyright Metropolitan Police Image caption CCTV captured the girls passing through security at Gatwick Airport

Ross Frenett, from the think tank the Institute for Strategic Dialogue, said the girls' long-term prospects were concerning.

"It seems likely they're going to get married pretty quickly and then probably settle into some kind of domestic life," he said.

"The worry is more what happens in the long term, because as [Islamic State] tends to lose territory, this idea of just settling down and helping it from the back end won't necessarily work any more.

Turkish criticism

"We could see women taking a more active role in this conflict and that's a real worry here."

Earlier, Scotland Yard denied it had taken three days to inform officials in Turkey about the schoolgirls travelling to Syria.

Turkey's Deputy Prime Minister Bulent Arinc said officials would have taken "necessary measures" had they known earlier.

Image caption The girls flew from London to Istanbul and are thought to have crossed into Syria at Kilis

But the Met has said it began working with Turkish authorities a day after the girls went missing.

Mr Arinc 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."

'Great assistance'

Scotland Yard said officers made contact with the foreign liaison officer at the Turkish Embassy in London on 18 February - the day after the girls left the UK.

"Since then we have been working closely with the Turkish authorities who are providing great assistance and support to our investigation," a spokesman added.

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.

UK police officers have also travelled to Turkey - although their role in the country has not been confirmed.

The three girls had been studying for their GCSEs at the school in Tower Hamlets, east London - where they have been described as "straight-A students".

A fourth girl from the school is believed to have travelled to Syria in December.

Bethnal Green Academy principal Mark Keary said there was no evidence they had been radicalised at school.