The trio, one aged 16 and two aged 15, who were not named, pleaded guilty to participation in a “terrorist” organisation at a trial conducted behind
closed doors in Vienna.

Bulgarian authorities arrested the 16-year-old in May on his way to Syria to join Isis, sending him back to Austria where he was put in custody but then released.

He then attempted to convince 10- and 11-year-olds to join the extremist organisation. He was given a 10-month sentence plus a further 20 months suspended.

The other two had concrete plans to go to Syria but were prevented from leaving in June, prosecutors said. One of them attacked his father with a screwdriver after he refused to give his son his passport.

He was also given a 10-month term and 20 months suspended and the other 15-year-old was sentenced to seven months with 14 months suspended.

All three, who were of Chechen and Turkish origin, were Muslims who were radicalised in just a few months, the court heard.

Like other European countries, Austria has seen a wave of people — 260, according to the government, many of them minors and some young women — leave the country to join Isis in Syria and Iraq.

Some have since returned. Around 40 of these are in custody and more than 80 are under close surveillance, according to the interior ministry.

In July a Muslim preacher known as Ebu Tejma at the centre of an Austrian jihad propaganda network was sentenced to 20 years in jail for “brainwashing” dozens of young people as young as 14.