Fellow biker Klaas Otto said the men were acting independently of their club
Three members of a Dutch motorcycle club with military backgrounds have gone to Iraq to help fight Islamic State (IS), a fellow biker says.
The three left for northern Iraq to help Kurds there after being horrified by news of IS atrocities, Klaas Otto told Dutch media.
All are trained soldiers who have served abroad in the past, he said.
Dutch prosecutors told BBC News that they were not necessarily breaking the law by fighting on the Kurdish side.
Wim de Bruin, a spokesman for the Dutch prosecutor's office, said that signing up with organisations like IS or the Kurdish militant group PKK was banned but joining a foreign armed force was no longer forbidden.
But if there was proof that they were committing murders or rapes then "of course, it would be a different story", he said.
The Netherlands' defence ministry said it could not be held responsible for choices made by ex-servicemen.
By Anna Holligan, BBC News, The Hague
The story emerged after photos began circulating on social media. One shows a man dressed in green military fatigues, clutching a Kalashnikov, sitting alongside a Kurdish fighter.
The Netherlands has a considerable Kurdish community. A few weeks ago hundreds demonstrated inside the Dutch parliament, voicing their outrage at the Netherlands' failure to intervene to protect Kurds under siege in the northern Syrian border town of Kobane.
Forty-nine dual nationals have had their Dutch passports revoked as part of the Netherlands effort to stop Islamists travelling to fight in Syria and Iraq.
So, the question is, why won't the Dutch bikers be subject to the same sanctions?
A spokesman for the Dutch prosecutor's office says there is no evidence the bikers are involved in the fighting or that they have joined a terrorist group.
The three men are members of an infamous motorbike gang, No Surrender, the biggest biker club in the Netherlands, our correspondent reports.
Mr Otto said they were acting independently, rather than as representatives of the club. He described one of them, whose name was given only as Ron, as a "good friend".
They apparently come from the Dutch cities of Amsterdam, Rotterdam and Breda.
"They are trained guys with lots of experience - with foreign missions, too," Mr Otto told Dutch broadcaster Omroep Brabant. "They are extremely disciplined. They don't drink any alcohol, not even on club evenings."
Talking of their motivation, he added: "They wanted to do something when they saw the pictures of the beheadings."
Images have appeared on social media of two European men posing with Kurdish fighters, apparently inside Iraq. There was no indication that they were using motorbikes.
BBC News - Dutch bikers join fight against Islamic State in Iraq
