BERLIN — In a twist to the foreign-fighter syndrome sweeping Europe, a handful of bikers are vowing to bring the pain to the Islamic State.

Already, the leader of the Dutch biker gang No Surrender has claimed that three of his peers have joined the fight against the Islamist militant group in the Middle East. But the ranks of “biker fighters” appear to be growing. Now, at least a couple of members of a German biker club say they, too, have signed up.

Why are they going? In Germany, at least, the move appears linked to the bikers’ ethnic backgrounds. The men in question hail from the Cologne chapter of the Median Empire Dark City Motorcycle Club, a group made up largely of ethnic Kurds. The club, founded in 2011, is named after an ancient empire that stretched from eastern Anatolia to India. The only difference between its modern-day members and the great warriors who once roamed ancient lands, the club states on its Web site, is that today “we are sitting on motorcycles instead of the backs of horses.”

The story of Germany’s fighting bikers has gripped the domestic press, particularly as the men post photos and updates on social media. One post shows two beefy bikers with machine guns slung over their backs as they climb hilly, arid terrain in biker gear.

The group’s self-described leader, who goes by the name Azad, posted an online appeal for funds and equipment donations in English: “We got Members in Kurdistan fighting against the terror Organization "Islamic State" ISIS !!” he wrote. “We need urgent Equipment like Night Visions, ABC Gas Masks, Bullet Proof Vests and much more !! Please support us to fight for humanity, love and peace!!!”

Just how dedicated everyone in the group is to love and peace, however, remains in doubt. Erich Rettinghaus, chairman of the police union in German state of North-Rhine Westphalia, where the men appear to have come from, said several of the group's members have been tried in connection with organized crime. He said the group's claims of fighting with the Kurds against the Islamic State also may be aimed at sending a message to rival gangs.

“They want to signal to other gangs, such as the Hells Angels or the Bandidos,” he said. “'You better watch out for us. We are damn tough.’”

The Germans followed a trail blazed by Dutch bikers. A leader of the secretive No Surrender motorcycle gang claimed last month that three of his members had left to help the Kurds, traveling to northern Iraq from the Dutch cities of Amsterdam, Rotterdam and Breda.

In an interview with Kurdistan Television, one of them appeared to be ethnic Dutch and said he was fighting because he was outraged by the atrocities he had witnessed on television. Giving his name only as “Ron,” he told an interviewer that once you see such images, “you can’t stay at home sitting on the couch.”

Dutch prosecutors have previously been quoted as saying that joining an armed group that is not a terror organization is no longer a crime under Dutch law. However, Wim de Bruin, a spokesman for the Dutch public prosecutor, said it was “a misunderstanding“ that Dutch citizens were free to fight against the Islamic State.

He said those who sign up for combat on foreign soil could still be charged at home for committing crimes — such as murder — that violate Dutch law. He conceded that such cases were “hard to investigate, because it’s far away and it’s difficult to find witnesses.”

But, he said, “if you join the fight against IS in Iraq and you commit a crime, you are quite likely to be prosecuted.”