British twin sisters are feared to have fled their home in the UK to join Isis jihadi fighters in Syria.
The 16-year-old girls from Manchester are believed to have crept out of their bedroom in the middle of the night and boarded a flight to Turkey.
The girls' parents found their beds empty at 8am last Thursday and called the police. Counter-terrorism forces were subsequently alerted when the girls contacted their family from Syria, where their elder brother is believed to be a jihadi fighter.
A spokesman for Greater Manchester Police said: "The girls flew from Manchester International Airport to Istanbul. Since their departure, the girls have been in contact with their family.
"We are attempting to confirm their current location and secure the wellbeing of both girls."
The twins are of Somali origin and their family are believed to have moved to the UK 10 years ago. The girls are believed to be strict Muslims and have reportedly told their family that they will not return home.
A police spokesman told Sky News: "It's unlikely that two 16-year-old girls would be able to afford two expensive flights so we are looking at the possibility that they have a contact there.
"It's definitely a line of enquiry that their trip could have been bankrolled by a third party, that it was pre-planned."
Last month, Isis, which now refers to itself as the Islamic State, released a video featuring five young jihadi fighters from the UK and Australia threatening to take their holy war to Jordan and Lebanon.
British student Nasser Muthana, 20, is heard saying in the video: "We have participated in battles in Sham and we will go to Iraq in a few days and we will fight there. We will even go to Lebanon and Jordan with no problems."
Muthana also posted a photograph online showing a stash of improvised explosive devices in a garage.
A missing 18-year-old from Coventry was also believed to have fled to Syria to join Isis last month, after he posted images of himself on Instagram holding guns and tweeting that he had joined the extremist Sunni militant group in Syria.
On Saturday, a 21-minute video surfaced online featuring a man purporting to be Isis' leader Abu Bakr al-Baghdadi, calling on Muslims to obey him and join the group's caliphate, which straddles parts of Syria and Iraq.
The video was reportedly filmed at the Great Mosque in the northern city of Mosul on Friday and was released on at least two websites known to be used by Isis.