ISTANBUL (Reuters) - A 19-year-old British woman, one of a group of medical students that includes seven Britons, an American and a Canadian thought to have travelled to join the Islamic State in Iraq and Syria (ISIS), has told her family she wants to go home, a Turkish lawmaker has said.

"A female student, 19-year-old Lena, sent a message to her family saying she wanted to go back. We will try tomorrow to bring her and those who are with her back, if we can persuade them," opposition CHP lawmaker Mehmet Ali Ediboglu said in a television interview late on Wednesday.

The medical students, aged between 19 and 25 and who include two from Sudan, flew from the Sudanese capital Khartoum to Istanbul on March 12.

A Turkish government official said the British, Turkish and Sudanese security services were jointly investigating but gave no further details.

Thousands of foreigners from different countries have joined the ranks of radical groups such as ISIS, many of them crossing through Turkey.

Turkey has stepped up border security, regularly releasing details of would-be fighters it has detained, after criticism it had not done enough to stem the flow of foreign fighters through the region.