Russia has sent three nuclear submarines to a naval parade for the first time in a "show of strength" after recent skirmishes in the Black Sea.
On Monday, a Russian Oscar-II Class nuclear-powered cruise missile submarine was spotted entering the Baltic Sea.
Two other nuclear-powered submarines were also identified, one likely to have been armed with nuclear missiles and the other an attack boat.
A nuclear ice-breaker and several powerful surface ships are also expected to attend the event in St Petersburg, planned for July 25.
HI Sutton, a defence analyst, said the annual naval parade was a display of strength but "this is a significant amount of firepower to put on show".
"Russia has been sending nuclear submarines to the naval parade in St Petersburg for several years, but this is the first time three submarines have been sent," he told The Telegraph.
"They are the only nuclear submarines in the Baltic. These deployments act as a show of strength for both home and foreign audiences. Nato and unaligned Baltic States will likely keep a careful eye on this submarine."
The inclusion of three significant submarines is likely to be in part a response to the recent incident in the Black Sea when the Royal Navy destroyer HMS Defender was confronted by Russian forces whilst transiting the internationally recognised maritime route between Ukraine and Georgia.
Russia said the passage of the British Type-45 air defence ship was a "provocation" as it passed close to Crimea, Ukrainian territory annexed illegally by Russia in 2014.
The Oscar Class submarine, designated K-266 Orel, passed under the Great Belt Bridge, which spans the entrance to the Baltic, at about 8.50am local time on Monday.
Russian Submarine – Oscar class II
It will be joined in St Petersburg by the K-549 Knyaz Vladimir, a Borei-A Class nuclear-armed submarine and the K-157 Vepr, an Akula-Class attack boat, which passed under the Great Belt bridge at 8.30am local time on Wednesday.
The Oscar-II Class boat is a large cruise missile submarine designed during the Cold War.
Its primary mission is to attack Nato’s Carrier Strike Groups, for which it is equipped with 24 P-700 Granit supersonic anti-ship missiles, designated by Nato as SS-N-19 SHIPWRECK. These missiles have a range of 340 nautical miles and can carry a 500 kiloton thermonuclear or 750kg conventional warhead.
Some Oscar-II submarines have been upgraded and may carry up to 72 smaller missiles in their place, but most are still armed with P-700.
The UK has four nuclear-powered and nuclear-armed submarines.