A senior Russian military official said on Monday that Russia was ready to open fire on foreign ships and submarines that illegally enter its territorial waters, the Interfax news agency reported.
Any such decision would, however, be taken only at the "highest level", Stanislav Gadzhimagomedov, deputy head of the main operational department of the General Staff, was quoted as saying.
The comment came two days after Moscow said a Russian naval vessel had chased away a US submarine in Russian waters in the Pacific. The United States denied it had carried out military operations in Russian territorial water
Tensions between the two countries are running high, with Washington warning that Russia could attack Ukraine at any time. Moscow denies any such intention, despite massing more than 100,000 troops near Ukraine's borders, and has accused Western governments of hysteria.
Interfax quoted Gadzhimagomedov as saying Russia was able to adequately counter the US fleet in the Black and Baltic Seas but did not have enough ships and plans to do so in the far east at the moment.
All of Russia's military fleets have been carrying out exercises in recent weeks across vast areas including parts of the Atlantic and Pacific Oceans and the Mediterranean.
More than 30 ships from the Russian Black Sea fleet have started training exercises near the Crimea peninsula as part of wider navy drills, RIA news agency reported on Saturday citing the fleet, as Western nations warned that a war in Ukraine could ignite at any moment.
Russia's military moves are being closely scrutinised as its massing of troops near Ukraine and a volley of hawkish rhetoric have rattled the West and sparked concern it plans to invade Ukraine. Moscow denies any plan to invade, saying it is maintaining its own security against aggression by NATO allies.
RIA said that more than 30 Russian ships have left the ports of Sevastopol and Novorossiisk "in accordance to the plans of the drills" near Crimea, which Russia annexed from Ukraine in 2014.
RIA said the aim of the drills was the defence of the coast of Crimea, the outposts of the Black Sea fleet as well as the economy sector and naval communications.
Russia announced last month its navy would stage a sweeping set of exercises involving all its fleets this month and next, from the Pacific to the Atlantic, the latest show of strength in a surge of military activity during the standoff with the West.