A spokesman for the Russian Foreign Ministry on Wednesday evening confirmed two US citizens had been arrested in the port city of Novorossiysk, home to a major Russian navy base, but refrained from giving additional details.
According to the Kommersant newspaper, David Gaaga and Cole Brodovsky were arrested while addressing a local branch of the Church of Jesus Christ of Latter-day Saints (Mormons) in English. The men were usually based in Rostov-on-Don, 200 miles to the north, the paper added.
In a strict reading of Russia's migration laws, prosecutors contended that such "English lessons" were not compatible with the religious visas the men had used to enter the country with. In court, the men insisted that discussions in English were part of their religious work.
Unsurprisingly, a district judge agreed with prosecutors, finding them guilty of violating migration rules inside a border zone. According to the judgment, the men were fined 2,000 roubles (approx £25) each and moved to a temporary detention centre ahead of their likely deportation.
Mormon missionaries continue to practice legally in Russia, having escaped the provisions of new "anti-terror" laws applied to many other "foreign" minority religious groups. Last month, a group of Jehovah's Witnesses in the Siberian city of Surgut complained of harassment and torture at the hands of Russian law enforcement.
Friday's arrests also come on the back of the December arrest of US citizen Paul Whelan over alleged espionage attacks. He spent his 49th birthday in prison this week, with little prospect of an imminent release.
A spokesman for the US State Department told CNN they were aware of the arrest, and "had no higher priority than the protection of US citizens abroad."