A Dutch court on Thursday convicted six men of belonging to a network recruiting young Muslims to join the Islamic State militant group (ISIS), handing them jail terms of up to six years.

Three other defendants received lesser sentences for other terror-related charges including a woman, who was jailed for seven days for posting a message on social media deemed as incitement.

"The criminal organisation aimed to incite and recruit 'brothers' to travel to fight in Syria and financed them to that end," Judge Rene Elkerbout said at a heavily-fortified courthouse on the outskirts of Amsterdam.

The complex 190-page verdict was handed down in a landmark case that experts say tested the boundaries of religious freedom and freedom of expression in the Netherlands, known for its tolerant values.

Eight men and one woman initially faced several charges including "belonging to a terrorist organisation", incitement to commit acts of terror, recruiting youngsters to join ISIS and undergoing military training in Syria.

The recruitment process took place through a "constant process of ideological ripening" which the judge said targeted vulnerable and impressionable youngsters, particularly in Schilderswijk, a poor, working-class neighbourhood of The Hague's inner city.

The suspects were arrested last year in a massive police probe into jihadist activities in Schilderswijk, once a traditional Dutch working class area, now inhabited by second and third-generation immigrants mainly of Moroccan and Turkish descent.

Public prosecutors said the group "formed a criminal and terror organisation."

"They all had different roles and eventually contributed to the aims of the organisation," prosecutors said in a statement.

A three-judge bench agreed and sentenced the main defendant Azzedine Choukoud, 33 -- whom they referred to as "the driving force" behind the group -- to six years in jail.

Two other men, aged 24 and 26, were tried and sentenced in absentia to six years in jail. Both are believed to be fighting in Syria.

Three others who formed part of the network were sent to prison for between five and three years, including Oussama Chanou, 19, whom prosecutors said was the group's "spiritual leader" and Rudolph Holierhoek, 25, dubbed the network's "media man."

Choukoud's lawyer Andre Seebregts denounced the sentences after the hearing as "very, very high".

"This is round one, there is a good chance that we will... appeal this," he told reporters outside the courtroom.

The judges said the network relied heavily on social media to find recruits, with one Facebook page containing at least 97 messages which constituted "incitement to travel and fight in Syria, including encouraging Dutch jihadists to fight in the armed struggle."

Mostly from Schilderswijk, the suspects maintained their innocence, claiming religious freedom and freedom of opinion allowed them to put out messages in support of ISIS.

The case comes two years after concerned parents in Schilderswijk filed missing persons reports with police, triggering the biggest Dutch probe to date into radical Islamist activities.

At the time, many of the accused were involved in pro-IS protest marches in the neighbourhood and also posting messages of support on Facebook and Twitter.

A spokesman for the public prosecutor's office, Wouter Bos, expressed satisfaction with the sentences, saying it would would serve as a deterrent for other groups planning to recruit youngsters to jihadist activities.

"This is a test case, which will set a precedent for the future," Bos said.

At least 220 Dutch citizens have left the Netherlands for Syria and Iraq to join fighters there, according to the Dutch National Coordinator for Security and Counterterrorism (NCTV).

About 40 have since come home, while another 42 are believed to have been killed in the fighting.