A Moscow court on Tuesday extended the pre-trial detention of a dual Russian-German citizen accused of treason in an ultra-secret criminal case.Russia has arrested a spate of Western citizens amid its offensive on Ukraine, drawing accusations that it is taking "hostages" to use as bargaining chips for prisoner exchanges.According to state-run news agency TASS, German Moyzhes, a migration lawyer who helped Russians apply for residence permits in Europe, was arrested in St.

Petersburg in May by Russian security services.He was then transferred to Moscow where he has since been held in pre-trial detention.A case record on the Lefortovo court website states that in a hearing on Tuesday, the pre-trial detention of "G.

Moyzhes," charged with treason, was extended.It provided no other details about his case and treason cases are classified in Russia.State media had not previously covered any details of his arrest or case.Moyzhes was well-known in St.

Petersburg as a local urban activist, where he was a prominent pro-cycle campaigner.Talk of a possible major prisoner exchange between Russia and the United States, possibly involving other Western countries, has ratcheted up in recent days.Last week a Russian court sentenced U.S.

journalist Evan Gershkovich to 16 years in prison on espionage charges rejected as baseless.U.S.-Russian journalist Alsu Kurmasheva was also sentenced in another secret trial to six and a half years in prison for breaching Russia's strict "military censorship" laws.The White House has called for both of them to be released and said negotiations for Gershkovich are ongoing.In exchange, Russian President Vladimir Putin has implied he is seeking the release of Vadim Krasikov, a Russian convicted in Germany of killing a Chechen former rebel commander in a Berlin park.A German court said the killing was an assassination ordered by the Russian state and the Bellingcat investigative outlet has reported Krasikov worked for Russia's FSB security service.