All 24 Ukrainian sailors, nabbed at a strait near Crimea last week, were moved to a Moscow pretrial detention centre on Friday.
The servicemen, including two security service agents, were arrested on Sunday, when three Ukrainian naval vessels in the Black Sea tried to enter the Kerch Strait to reach the Sea of Azov.
Russia's Investigative Committee has accused the Ukrainians of illegal border crossing.
In a tit-for-tat, Ukrainian President Petro Poroshenko has forbidden Russian male citizens from ages 16-60 to enter the country.
Russia and Ukraine have been at loggerheads since 2014, when Russia annexed Crimea after a controversial referendum.
Turkey, as well as the UN General Assembly, viewed the annexation as illegal.
Ukraine has also blamed Kremlin for separatist violence in eastern Ukraine, near the border with Russia.