Police have arrested two teenagers and opened a criminal inquiry into their alleged trip to Syria to join the Islamic State militant organisation.

A Winterthur prosecutor confirmed that the two suspects were detained at Zurich airport upon their arrival from Turkey on Tuesday.

The two suspects had apparently left Switzerland last December.

They are investigated for their alleged support of Islamic militant groups, banned under Swiss law.

The city of Winterthur has made the headlines in the past few months amid reports that young Muslims joined the jihad in Syria after being encouraged by radical preachers at a mosque east of Zurich.

The president of the An’Nur mosque denied it is his job to control the preachers.

Currently some 70 cases of jihadi radicalisation are being investigated in Switzerland, with criminal proceedings underway in more than 20 cases.

In November, the head of the Federal Police Office, Nicoletta della Valle, outlined plans for a proposed travel ban for radicalised jihadis, saying it would be possible to confiscate passports and require them to regularly check in with authorities.

In an interview with the Neue Zürcher Zeitung newspaper, della Valle said these measures and others were being considered in order to prevent and address jihadi radicalisation within Swiss borders.

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