British men are among a group of international volunteers preparing to fight against Turkish-led forces in north-west Syria, the BBC understands.
They have joined the Kurdish militia, the YPG, to fight the Turkish offensive on the Kurdish-held enclave of Afrin.
Among the volunteers is a 24-year-old British-Chinese fighter from Manchester, known as Huang Lei.
Lei, who originally travelled to Syria in 2015 to battle against IS, told the BBC it was his "duty" to fight.
The YPG has released a video showing an international group of volunteers who previously fought in Syria against so-called Islamic State.
Lei has told the BBC that there are two other Britons with him among the group, although they have chosen to remain anonymous.
In the video, an American 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," he says.
"We want to go there to help people defend the city and protect the people."
This marks a significant shift in the involvement of international fighters in the Syrian war.
With the IS threat diminished, for some volunteers their mission has changed.
"Defending Afrin is our duty," says Lei, who was a student studying international politics at the University of Manchester when he travelled to Syria in 2015.
Turkey views the YPG as terrorists 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 says: "For me personally, the kindness and comradeship the Kurdish people have shown is my motivation to stand against Turkey".
The British Foreign Office has repeatedly warned people not to get involved in fighting abroad, so signs of changing motivations from western anti-IS fighters may prompt fresh concern.
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."
Lei accepts he may face consequences if he comes back to Britain.
"I really hope I can return, but I don't want to come back and get arrested," he says.
"I am here to fight against terrorists. I don't want to come back home and become a terrorist."
'Afraid of death'
Lei has survived battles against IS, 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.
Does he fears this prospect?
"Of course I am afraid of death," he says. "But, on a basic level, so is every human fighting in the YPG."