A Dutch court has convicted four men in absentia for travelling to Syria to join extremists, including the Islamic State group, and sentenced them to six years in prison.

All four suspects convicted Friday by The Hague District Court are believed to be in Syria, where some 260 Dutch citizens have gone to fight in recent years.

In the written verdicts, the judges say the convictions should act as a deterrent to others considering travelling to Syria to fight with IS and other extremist groups.

The men, whose ages ranged from 23 to 29, left the Netherlands in 2012 and 2013 to join Islamic State and al-Qaida offshoot Nusra Front.

They were charged with "membership in a terrorist organization" and "preparing terrorist crimes" for their activities in Syria.