An American soldier arrested in Russia's far eastern city of Vladivostok on charges of stealing lost an appeal against his detention and will remain in custody, Russian state news agency RIA Novosti reported Friday, citing court officials.

The soldier, identified by court officials as Gordon Black, will remain in custody at least until July 2, the report said, after the Primorsky Regional Court upheld the lower court's ruling to place Black in custody pending investigation and trial.

Several U.S. officials said earlier this month that Black, a 34-year-old staff sergeant, was stationed in South Korea and was in the process of returning home to Fort Cavazos in Texas. Instead, officials said that Black, who is married, traveled to Russia to see a longtime girlfriend. He was detained in Vladivostok, a major military and commercial Pacific port in Russia's Far East, and accused of stealing from her.

RIA Novosti said, citing local police, that Black has admitted guilt and is cooperating with investigators.

Black's arrest further complicates U.S. relations with Russia, which have grown increasingly tense as the war in Ukraine drags on.

Russia is known to be holding a number of Americans in its jails, including corporate security executive Paul Whelan and Wall Street Journal reporter Evan Gershkovich. The U.S. government has designated both as wrongfully detained and has been trying to negotiate for their release.

Others detained include Travis Leake, a musician who had been living in Russia for years and was arrested last year on drug-related charges; Marc Fogel, a teacher in Moscow, who was sentenced to 14 years in prison, also on drug charges; and dual nationals Alsu Kurmasheva and Ksenia Khavana.