The police questioned Rhee Keun, a former Navy special warfare officer turned YouTuber, for violating the South Korean passport law following his return from Ukraine, where he had volunteered in an "international legion" of fighters opposing the Russian invasion.

According to law enforcement on Monday, the 38-year-old voluntarily appeared for questioning at the Seoul Metropolitan Police Agency last Friday.

Rhee reportedly admitted to most of the charges he faces and the police are expected to transfer his case to the prosecution within the week.

In early March, the former Navy officer posted on his social media that he had departed to fight for Ukraine through an unofficial route, despite the government's discouragement.

Officials in Seoul had warned that citizens who depart for Ukraine, currently under a travel ban amid the Russian invasion, without state approval can face up to a year in prison or a maximum fine of ten million won.

After reportedly suffering injuries in the ongoing war, Rhee returned home through Incheon International Airport on May 27.