The Turkish army says nine Britons have been arrested after allegedly trying to illegally cross the border into Syria.
The three men, two women and four children all with British passports were detained by soldiers at a military outpost at Ogulpinar.
They were trying to sneak across the border, said a senior Turkish official, and were taken into custody in Hatay province.
The nine are now believed to be awaiting deportation.
The UK Foreign Office has said it is looking into the reports that they attempted to enter the conflict-hit country.
UK security services estimate 600 Britons have gone to Syria or Iraq to join militant groups.
They including three teenage girls from east London, who are now believed to be in the Syrian city of Raqqa, an Islamic State stronghold.
Bethnal Green Academy pupils Shamima Begum, Amira Abase and Kadiza Sultana were last seen on CCTV at a bus station in Istanbul on 17 February.
It is understood they were following another 15-year-old girl who travelled there in December.
Several terror suspects have been successfully captured in Turkey before they have managed to cross the border.
They include three teenage boys, who have since been arrested by the Metropolitan Police on suspicion of terrorism offences.
It comes as a former chief prosecutor said headteachers do not want to inform police of concerns students may flee to Syria for fear the pupils are criminalised.
Two school leaders have confessed to being "scared" not all of their pupils will return after the Easter holidays amid fears they may travel to the war-torn region to join IS, Nazir Afzal said.
One principal said he has been approached by worried parents almost every day, Mr Afzal claimed.
Earlier this week, Syrian President Bashar al Assad said jihadists who travel to his country to join IS are being "logistically and militarily" supported by Turkey on a daily basis.
He has accused his Turkish counterpart, Recep Tayyip Erdogan, of ignoring would-be terrorists as they attempt to cross the border between the two nations.
Among the Britons in IS ranks are Mohammed Emwazi, also known as "Jihadi John", who has appeared in several beheading videos.
