Kadiza Sultana, 16, Shamima Begum, 15, and Amira Abase, 15
Kadiza Sultana, 16, Shamima Begum, 15, and Amira Abase, 15 have reached Syria.Met Police

Three missing British schoolgirls who left Britain to join Islamic State are believed to be staying at a house in al Raqqa.

Shamima Begum, 15, Kadiza Sultana, 16, and Amira Abase, 15, who flew to Istanbul from Gatwick Airport last month, have crossed into Syria, Sky News reports.

Speaking from the Turkey-Syria border, Sky's Stuart Ramsay said: "We're being told tonight that they've crossed into Syria through a crossing... which is not too far from Kilis.

"They're now apparently in a house that is owned or controlled - or at least hosted by - a British girl who had been in contact with them through the internet, and had brought them through Turkey and into Syria.

"We are told by very, very good sources within the city of Raqqa that they are there, that they are safe, and that is where they are staying ... we are told they are now inside Syria, they are inside al Raqqa, and they are under the control of Islamic State."

Begum and Abase, both 15, and Sultana, 16, were last seen February 17, when they boarded a flight from London's Gatwick Airport headed for Turkey, prompting fears that the teenagers were attempting to join the Islamist militants.

Last week, CCTV footage emerged which appeared to show the three teenagers at a bus station in Istanbul. They then boarded a bus to Gaziantep, a Turkish border town that has gained notoriety for being portal for foreign fighters traveling to Syria.

British authorities admitted mistakes in their investigation into the schoolgirls, after their families complained police should have contacted them during an earlier investigation into the disappearance of a classmate.

Scotland Yard had met with Shamima Begum, Kadiza Sultana, and Amira Abase about the disappearance of a classmate at their school, Bethnal Green Academy in east London. The girls were given letters by investigators to take home that asked their parents for permission to take statements, but the girls hid the letters, the BBC reports.

Halima Khanon, Sultana's sister, told the BBC that if the family knew about the investigation, they would have "taken precautions with my sister."

"With the benefit of hindsight, we acknowledge that the letters could have been delivered direct to the parents," the police said in a statement.

The trio left their homes in east London after telling their families they would be out for the day.