Among the volunteers is a 24-year-old British-Chinese fighter from Manchester, known as Huang Lei.
Mr Lei, who first travelled to Syria in 2015 to join the fight against ISIS when he was a student studying international politics at the University of Manchester, told the BBC: “Defending Afrin is our duty.”
He said there were two other Britons among the group but they wanted remain anonymous.
In a video released by the YPG, a man with an American accent says: “We’re all ready to go and fight in Afrin, against the invading force of Turkey.
“We’ve been training for a significant amount of time. We are prepared, and we have been supplied by the YPG to fight against the Turkish terrorists.
“We were fighting against ISIS in Syria and suddenly we heard that Turkey is attacking Afrin and bombing the city.
“We want to go there to help people defend the city and protect the people.”
Security experts believe this marks a significant shift in the involvement of international fighters in the Syrian war.
With ISIS all but defeated, the missions of many of the volunteers have changed.
Michael Stephens, a Middle East analyst from the Royal United Services Institute, said: “Previously their actions had aligned completely with the goals of coalition partners.
“Now these volunteers are taking on a sovereign country and a NATO ally and this brings up certain legal considerations.”
Turkey views the YPG as a terror group and has launched a new offensive to drive Kurdish fighters out of Afrin.
Lei says there is now a group of around 20 international volunteers who will go to defend the city, including Britons from Manchester, London and Leeds, and others from France, Germany, Spain and the US.
He said: “For me personally, the kindness and comradeship the Kurdish people have shown is my motivation to stand against Turkey.”
Mr Lei accepts he may face consequences if he comes back to Britain.
He said: “I really hope I can return, but I don’t want to come back and get arrested.
“I am here to fight against terrorists. I don’t want to come back home and become a terrorist.”
Mr Lei has survived battles against ISIS but a fight against the Turkish military may place him in even greater danger.
The Turkish army have been shelling Afrin, killing civilians as well as YPG fighters.
He said: ”Of course I am afraid of death. But, on a basic level, so is every human fighting in the YPG.”