FRANKFURT - A 20-year-old German man was sentenced to 3 years and 9 months in prison on Friday after he admitted joining Islamic State militants in Syria, in the first trial in Germany of a home-grown jihadi accused of membership of the insurgent group.

Judges decided to treat the man, known as Kreshnik B., who was born in Germany to Kosovan parents and spent six months in Syria last year, as a juvenile because of his lack of maturity. They said there was no evidence he had participated directly in fighting.

"As a youth he was not able to resist the influence of his Islamist friends," said chief judge Thomas Sagebiel.

Although he still displayed the attitudes of a radical Islamist, the judge expressed hope that a youth sentence would have the necessary educational effect.

In Germany, people between the age of 18-21 can be treated according to juvenile law if they are judged to lack an adult's sense of responsibility.

The sentence was 6 months below what prosecutors had sought but above the three years and three months sought by his defence lawyer after Kreshnik B., in exchange for more lenient treatment by judges, admitted joining IS and receiving weapons training.

He looked relaxed as he entered the court and looked down at his hands as the verdict was read. He was dressed casually and sported a full beard.

Islamic State militants have captured large swathes of territory in Syria and Iraq, beheaded Western prisoners, killed many non-Sunni Muslims and declared a caliphate governed by sharia law in the heart of the Middle East.

Thousands of Western volunteers have travelled to Syria and Iraq to join militant groups, raising fears in Europe and the United States of attacks by returning fighters, and authorities are cracking down on them and on fundraisers and recruiters.

About 550 German citizens have joined Islamic State in Iraq and Syria and about 60 have been killed, some as suicide bombers, authorities say. Some 180 are believed to have returned to Germany. Prosecutors are investigating about 300 others.

French judges last month sentenced a returning jihadi to seven years in prison, in the first verdict of its kind there, even though he spent only 10 days in Syria and did not fight.

Britain has sentenced two brothers to four and three-and-a-half year prison terms for attending training in Syria.

Some politicians and legal experts have argued that tough sentences may only build resentment and say more deradicalisation programmes are essential to countering the waves of youngsters leaving to fight in the region.