Turkey claims to have arrested nine Brits after catching them attempting to cross the border into Syria.
The Foreign Office said it is investigating reports that Turkish authorities detained the suspects as they tried to cross the border into the war zone.
A spokesman said: "We are aware of the reports and are looking into them."
The claim was made by Turkey's armed forces, who say they caught the nine Britons illegally attempting to cross the border.
It follows the arrest of at least four young men and women at the Turkish-Syria border last month, and after police launched an international manhunt for three schoolgirls in February.
Bethnal Green Academy pupils, Shamima Begum, Amira Abase and Kadiza Sultana are now believed to be inside Syria after flying to Turkey on a well-trodden path to the country.
It is understood they were following another 15-year-old girl who travelled there in December.
The Metropolitan Police believe around 600 Britons have travelled to Syria and Iraq since the conflict began, while around half are believed to have returned to the UK.
Prime Minister David Cameron, speaking to the BBC on the threat of Britons leaving to join ISIS, said: "In the past the danger has been more about boys but we can now see that is changing.”
"We can't entirely say it's a matter for the police or the border force, everyone has to be involved in spotting signs of radicalization and combating those signs.”
"It is a really big issue for our whole country to get to grips with and work out how we put a stop to it."
