Western volunteers prepare to fight against Turkish forces in Afrin

“We are prepared and have been supplied by the YPG to fight against Turkish terrorists,” said one American volunteer. “It wasn’t really something that was unexpected. It’s been going on for some time - Turkey has been helping [IS] fight against the Kurds,” he added.

The 20-person group is calling itself the Tabûra Şehîd Rûbar.

Amongst the volunteers is twenty-four-year-old British-Chinese fighter Huang Lei. Lei previously travelled to Syria in 2015 to fight against the Islamic State, telling the BBC it was his “duty”. 

"There was a desire on the part of the foreign fighters who fought in Raqqa and who are fighting in Deir az-Zour to go to Afrin," senior SDF official Redur Xelil later told Reuters.

He declined to say when the foreigners had gone to Afrin, but said they numbered in the "tens". "They will wage battles against the Turkish invasion," Xelil said.

"There are Americans, Britons, Germans, different nationalities from Europe, Asia and America," Xelil said.

Although international fighters have been involved in the Syrian war for years, this marks a different form of involvement – and entails fighters taking on a sovereign country and NATO member. “This brings up certain legal considerations,” said Michael Stephens, a Middle East analyst from the Royal United Services Institute, in a BBC interview.

Turkey began Operation Olive Branch on Saturday – an air and ground campaign to root out the People’s Protection Units (YPG), a Kurdish militia Turkey considers a terrorist group, in Afrin.

The YPG has a contingent of 8,000 to 10,000 fighters in the 800,000-person city.

Dozens have been killed and at least 5,000 civilians displaced since the operation began last week.