Nine British medics feared to have crossed into Syria
Nine British medical students and doctors are feared to have travelled to Syria to work in areas controlled by Islamic State militants.
The group, all in their late teens or early 20s, had been studying medicine in Sudan.
Some of their relatives, who have travelled to the Turkish-Syrian border, told the BBC they were gravely concerned about their welfare.
The medics are believed to have entered Syria more than a week ago.
They have been named as Ismail Hamadoun, Tamir Ahmed Abusibah, Mohammed Osama Badri, Hisham Muhammed Fadlallah, Ahmed Sami Khider, Nada Sami Khider, Lena Mamoun Abdelgadir, Rowan Kamal Zine El Abidine and Tasneem Suliman.
BBC Turkey correspondent Mark Lowen said some in the group had sent messages to their parents saying they were doing voluntary work to help Syrians.
Our correspondent, who described the group as British-Sudanese, said their families had travelled to Gaziantep in southern Turkey to try to find them.
One of the fathers said he was not getting enough support from British and Turkish authorities and did not understand how Turkey allowed such a large group to cross the border.
The Foreign Office said it was providing consular assistance to their families.
BBC Turkey correspondent Mark Lowen
The agonising wait goes on for the British-Sudanese families hoping for news of their children.
The father of one of the missing medics has told me he no longer holds out hope that they are still in Turkey and is convinced they've already crossed into an area of Syria controlled by so-called Islamic State.
The group is 13-strong, mainly British-Sudanese and were studying at a faculty in Khartoum, where the fear is they may have been radicalised and recruited by IS.
The nine Britons are understood to have been joined by two Sudanese medics, one who is American-Sudanese and one Canadian-Sudanese.
The Observer newspaper reported that the students' parents believed they were in Tel Abyad and wanted to work with IS, but they were almost certain they did not plan to take up arms.
The medics are said to have been born in England but went to Sudan to study and experience "a more Islamic culture".
A friend of Tasneem Suliman in Khartoum told the BBC the young woman and her friends had "radically changed" a few years ago and started wearing the full veil, which "shocked a lot of their friends" in the British Sudanese community.
A Home Office source told the paper the medics would not automatically face prosecution under anti-terror laws if they returned to the UK if they could prove they had not been fighting.
Correction 23 March 2015: An earlier version of this story named one of the medics as Mohammed Elbadri Ibrahim. This has now been changed to Mohammed Osama Badri.