Two Russian warships transited the Bosphorus en route to the Black Sea on Saturday and 15 smaller vessels completed a transfer to the sea as Moscow beefs up its naval presence at a time of tense relations with the West and Ukraine.
The reinforcement coincides with a huge build-up of Russian troops near Ukraine, something Moscow calls a temporary defensive exercise, and follows an escalation in fighting in eastern Ukraine between Russian-backed separatists and Ukrainian government forces.
Russia's relations with Washington, which cancelled the deployment of two of its own warships to the Black Sea last week after fierce Russian protests, are at a post-Cold war low.
Moscow expelled 10 US diplomats on Friday in retaliation for the expulsion of the same number of Russian diplomats from the United States over alleged malign activity.
Russia has also temporarily restricted the movement of foreign warships "and other state ships" near Crimea, which it annexed from Ukraine in 2014, a move condemned by both Kyiv and Washington.
Two Russian Ropucha-class landing ships from Russia's Northern Fleet, capable of carrying tanks and of delivering armour and troops during coastal assaults, transited the Bosphorus on Saturday, a Reuters reporter in Istanbul saw.
More Russian naval reinforcements in the form of two more landing ships, this time from Russia's Baltic Fleet, are expected to imminently transit the Bosphorus.
The RIA news agency on Saturday also reported that 15 smaller vessels from Russia's Caspian Flotilla have completed their transfer to the Black Sea as part of an exercise.
In a further sign of heightened tensions in the region, a ship carrying logistics trucks and equipment for NATO forces in Romania transited the Bosphorus on Friday evening, the same Reuters reporter saw.
In St Petersburg, Russia's FSB security service briefly detained a Ukrainian diplomat, Ukraine's foreign ministry said on Saturday.
The Interfax news agency earlier cited the FSB as saying Oleksandr Sosoniuk was taken into custody when he tried to obtain classified information from Russian law enforcement databases during a meeting with a Russian citizen.