LONDON — The father of a British citizen who fought with Kurdish forces in northern Syria has been arrested on suspicion of financing terrorism, in a rare action taken against a relative of a European citizen who joined the Kurdish ranks.

The father, Paul Newey, said on Friday that he had given £150, or about $196, to his son, Daniel, who traveled to Syria in October to fight with the People’s Protection Units, also known as the Y.P.G. The Kurdish force fought with the American military against the Islamic State, or ISIS, in Syria.

Daniel Newey first went to Syria in August 2017. He returned home in March 2018, before going back to Syria this fall after Turkey launched an offensive in Kurdish-controlled territories.

Paul Newey, 49, said he had not known his son had returned to Syria when he sent him the money. “I transfer money to him all the time, and I wanted to get him back on the road after all he’s been through since he’s been back,” Mr. Newey said in a telephone interview.