WASHINGTON (AP) — An American soldier visiting a girlfriend in Russia’s port city of Vladivostok was arrested on charges of stealing from her and remains in custody, according to several U.S. officials.

U.S. officials said Monday the soldier, Staff Sgt. Gordon Black, 34, 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. His arrest only further complicates U.S. relations with Russia, which have grown increasingly tense as the war in Ukraine drags on.

The officials spoke on condition of anonymity to discuss personnel details.

Cynthia Smith, Army spokeswoman, confirmed that a soldier was detained on Thursday in Vladivostok, a major military and commercial Pacific port, on charges of criminal misconduct. She said Russia notified the U.S. and the Army told the soldier’s family.