The trio left Britain several days ago, and they were detained in Istanbul after British officials notified Turkish authorities. Police believe the teens were planning to join extremists in Syria.

The three were arrested on suspicion of the preparation of terrorism acts and were in custody at a central London police station, police said. Their names haven't been released. Two are aged 17 and one is 19.

They are believed to be the latest in a growing number of Britons trying to travel to extremist-held territory inside Syria. Last month, three British schoolgirls left the U.K. for Turkey and, police believe, crossed the border into Syria to join Islamic State militants.

British legislator Keith Vaz, chairman of the Home Affairs Select Committee, said the latest case shows that the number of young Britons trying to reach the conflict zone in Syria "is on a much larger scale" than had been thought.

He praised Turkish authorities for acting quickly to detain the teens before they entered Syria. Similar attempts to stop the schoolgirls were unsuccessful.

British counterterrorism officers learned on Friday the teens had gone missing from their homes in Britain and they were thought to be traveling to Syria, police said.

An increasing number of Britons have traveled to Turkey to use it as a crossing point to enter Syria and to join Islamic State extremists who control territory in both Syria and Iraq. The militants have declared an Islamic caliphate on their territory.

British police say roughly 700 Britons have traveled to Syria to join extremists, raising concerns that some may launch attacks inside the U.K. if they return. Recent cases indicate a growing number of young women traveling there to become "jihadi brides."

Authorities say Internet-based social media have made it much easier for young Britons to communicate with extremists inside Syria.