Oslo, May 9 (IANS): A Norwegian court has sentenced three men to jail after they were convicted of joining or supporting the Islamic State (IS) extremist group in Syria, a media report said.

It is the first time a Norwegian court has dealt with a case in which Norwegian citizens are accused of having participated in combat operations for the IS group in Syria, Xinhua reported on Friday, citing Norway's public broadcaster NRK.

Valon Avdyli, 28, was sentenced to four years and nine months in prison, while Djibril Abdi Bashir, 30, was handed a jail term of four years and three months for having participated in combat operations for the IS group in Syria.

Visar Avdyli, 25, a brother of Valon, was sentenced to seven months in prison for supporting the IS group and acquiring equipment for a third brother who was killed while fighting for the extremist group in April last year.

All three men had pleaded not guilty during the trial and lawyers for the Avdyli brothers said they would appeal the court ruling.

Norway's intelligence service chief had said in March that the Nordic country must be prepared for possible terrorist attacks as more people have travelled from the country to fight in Syria and Iraq than it knows.

"We are reasonably confident that about 70 people have travelled. We assume that the real figure is significantly higher and the number continues to rise," Benedicte Bjoernland, the head of the Norwegian domestic intelligence service PST, said.

About 15 people have been killed in the fighting while about 25 have, so far, returned to Norway, she added.

The experience those fighters gained would lower the threshold for violence when they return home, the PST chief said, adding that their peers who did not travel could also pose a threat.