A senior source at Serbia's interior ministry had told Reuters earlier that Vassilev has handed himself in to Serbian authorities.

"Vassilev presented himself to Serbian authorities", Konstantin Simeonov, a lawyer for Vassilev, told Reuters by phone. "From there, an extradition process will start. The Serbian court has 40 days to decide on the process," he said.

(Reporting by Tsvetelia Tsolova; Editing by Matt Robinson)