FRANKFURT (Reuters) - A 20-year-old German man charged with joining Islamic State of Iraq and Syria (ISIS) militants in Syria last year went on trial in Frankfurt on Monday, the first-such case in Germany.

Kreshnik B., who was born in Germany and whose parents are from Kosovo, is charged with membership of ISIS between July and December 2013.

German prosecutors say he travelled to Syria with others to fight and once there, received weapons training, fulfilled guard and medical duties and took part in a recruitment campaign close to the city of Aleppo. They say he engaged in combat on three occasions.

Kreshnik B. was arrested at Frankfurt airport on his return in December 2013 and has been held in pre-trial custody since.

His lawyer, Mutlu Gunal, told the court that "the accused is suffering from post-traumatic stress", having probably witnessed much civilian suffering.

Dressed in a black hooded jersey, black T-shirt and grey tracksuit trousers, Kreshnik B. smiled as he entered court. "You're a very young man, without a significant criminal record," lead judge Thomas Sagebiel said. "We consider giving you a chance to help us exercise leniency," he said, adding this required Kreshnik B.'s full cooperation. "It mostly depends on you."

The group has captured swathes of territory in Syria and Iraq, changed its name to the Islamic State and declared a caliphate or Islamic state ruled under syarish law in the heart of the Middle East.

The United States has unveiled a plan to fight the militants, leading an international coalition, and on Monday, French President Francois Hollande opened a conference in Paris bringing together officials from about 30 nations to discuss tackling the jihadist threat.

Thousands of Western volunteers have travelled to Syria and Iraq to join militant groups including ISIS. The trend has raised fears in Europe and the United States of attacks by returning fighters.

Security authorities say about 400 German citizens have joined ISIS in Iraq and Syria and about 40 have died, some in suicide attacks. More than 100 are believed to have returned.

Prosecutors said they did not believe Kreshnik B. had planned any attacks on his return.

Germany last week announced a ban on ISIS, making ISIS propaganda, symbols and activities illegal. Austria said on Monday it planned a similar crackdown.

Chancellor Angela Merkel has broken with Germany's post-war policy of not sending arms to war zones and has begun to send weapons and equipment to Iraqi Kurds fighting the militants.